I'm curious what the community thinks is the best buying order for the various splat books (not adventures) after the core rulebook?

For my own opinion, I'd say Battlefleet Koronus is a very good, important, all-around book. It adds some nice ships, a couple new classes of them, and a handful of very potent ship upgrades. If you have read any threads on here about the sheer oomphatude that certain uses of Attack Craft or Torpedoes, two things the core book says Rogue Traders rarely employ, due to the expense, and focus away from their more money-making enterprises, this is the book that gives you rules for them; nova cannons, too. It's probably my favorite book, because it is also full of fun fluff, and more than a few alien ships, this time of numerous races, to throw in the path of your Exploroers.

Into the Storm and Hostile Acquisitions are also good, because they give you some nice character options, including most of the Alternate Career choices, and expanded, more "flavored" gear for them to use. ItS also has more ship options, while HA does not, but has more ship upgrades, which are about as good. HA is also a nice book for your players who are not always the best of people; your Warrant gives you much heady power and leeway, in a place where the law is weak against a man who can bombard planets from orbit. This book helps describe the bad things good players might want to try.

I like Navis Primer, but you can take it or leave it. It's full of new things for Navigators and Astropaths, but between some of the Alt Career paths, the modified warp travel tables, and such, the value of it can vary greatly, and you, or your GM, might not care to use it.

Koronus Bestiary is a very good book, to me, filled with creature write-ups, both alien and iconic. It covers both beasts and people, so if you've been waiting for "official" Aspect Warriors, here you go.

Stars of Inequity you might just skip. In my paging through it, the colony rules are iffy, and maybe not assembled by the best Magos Calligrahi they could've chained to the bench. There might be a number of things you want to do with a colony, but the book might not let you, might not explain it well, if it does let you, or might not inform you that your colonies aren't the worlds for that, depending on your take of things, in 40k.

Faith and Coin is also here. It might be better written, but it had little in it that made me go ooh! Some of the relics are cool, but usually not much better than already introduced "regular" treasures, you might've already found. Much of what is nice in it, at least to me, is the fluff, and that can be good, if your party is a more pious group, or if you need it to make some NPCs who keep showing up on the player's doorstep seem more so.

For me, and this is purely my opinion, only, I'd say start with the corebook, then grab the Bestiary and Battlefleet Koronus, just to spread out the options that the GM has available. Once the game is going, Into the Storm and Hostile Acquisitions are good purchases to flesh out the party, as they grow in wealth and xp. Look into Navis Primer, at this point, and decide if you need more for your warp-touched characters, and if you want the advantages, and the hassle, of the modified warp travel system. Stars and Faith, I believe you could mostly ignore, and not hurt yourself, but if you "gotta catch'em all!", then they both have some good material in them, too.

For splats, I only have Lure and Edge of the Abyss. I love Lure, for the most part, but like many things FFG wrote, it is written as something a party could use as a first adventure, and it does have a bit of a rails feeling to it; while it has numerous options, and numerous optional side-bits, many of which could be fun post-adventure excursions, you definitely might want some of the other books to round it out, and bump it up, especially if your party has already been busy and successful. Nothing in it is not from the core book, or itself, so if you have a Grand Cruiser, or Attack Craft, even the best, most militant a-hole competing Trader might seem a cake walk. Edge has a nice adventure, but I like it more for the personalities it introduces, which is why I sometime don't think of it as an adventure book. You get some important Orks, like Morgaash and his battleship, da'Wurldbreaka, some powerful Eldar, a number of other things, and write-ups, maybe guidelines, at least, if you don't favor statted NPC blocks, for some of the Expanses OTHER most important Rogue Traders. For not giving your players much for new options, it gives your Expanse some cool additions to use.

Babble concluded. I hope that helped, and if anyone feels like conflicting with many of my takes on the books, your turn

Battle fleet koronus is a must have for any game that has more than a passing interest in ships. Good range of NPC ships from different races

Beastery has a very gstrong list of races including their weapons and philosophy. Rules for xeno generation

Stars of inequaty is worth picking up, just be prepared to add in a house rule or two. System, planet, treasure generation plus rules/guide lines for colonies

Into the Storm augments the core rules

Into the Storm and Battlefleet Koronus would probably be my first buys as they bring in a lot of options for character development, items, ship construction and combat and more. The Bestiary is a beautiful book with plenty of opponents or allies and some plot hooks, and actually I like Edge of the Abyss for the added information on the factions and enemies of the Expanse. Hostile acquisitions is good, and the nemesis idea is interesting, but I wouldn´t put it at quite the same level.

The adventures are atmospheric as set pieces, but railroady at times. The players have a goddamn starship the size and population of a habspire, and with enough weapons to make the government of a small planet tremble. It is fairly hard (and ingame, unsafe) to make them do something they don´t want to do.

• Into The Storm - If you decide to use one of the homebrewed character creation systems and ignore a lot of fluff. If you have a Dark Heresy rulebook you could basically run Rogue Trader with that and Into The Storm.
  
• Battlefleet Koronus - You are not interested in doing space combat or ship life.
  

1. Koronus Bestiary - Easily my favourite supplement. Rules for monster creation, lots of pre-made monsters that are interesting, expansion of existing Xenos races, and the flavour justifications for the monsters are great. You can basically write adventures from this bok alone.
   
2. Faith and Coin - A little bit of balance problem (though not nearly as bad as Navis Primer) and a lot of background expansion, but all in all it has a little bit of everything you could want excepting Starship components. Have not tried the adventure at the back.
   
3. Navis Primer - We had a Weirdboy in the campaign for awhile, and it was a lot of fun. Even without that, the expansion of Navigators/Astropaths/heretics is pretty interesting, and there are some quite challenging foes to use in here as recurring adversaries. Unfortunately the Psychic Power book is also the one with the most power creep. Obviously.
   
4. Hostile Acquisitions - Like Faith and Coin it has a little bit of everything you could want, but it seems to fall a little flat for me. I can't find anything wrong with it, it just didn't connect.
   
5. Stars of Inequity - Systems for creating worlds and treasure for your explorers, and some moer background groups, but it isn't as essential as the others I find.
   
6. Soul Reaver - It's a supplement that is Dark Eldar focused, which is a topic you can search and not comment on here. I've put it near the bottom, and we'll move on.
   
7. Edge Of The Abyss - One of the earliest splat books, and it's all the way down here because everything it does is done later and better by other books.
   

I wasn't going to post on this one. Venkelos gave you good advice, if for different reasons than I would have said, and I was certain Erathia was going to fill in the holes (as I perceived them). I've decided to post anyway. I like to hear myself type. With that caveat, I'll give you my votes with little explanation.

I'm not fond of Alternate Career Ranks. It can all be done with Elite Advances, and the special abilities they give bring power creep to the game. So while I'd not recommend any books for having them, I won't reduce the value of a book for having them.

For purposes of running a game, I'd get Into the Storm, Battlefleet Koronus, and Hostile Acquisitions . They all have equipment (shiny toys are a primary concern of any scifi game), both personal and starship components.

For purposes of world building and adventure creating, I'd get Koronus Bestiary, Stars of Inequity, and Edge of the Abyss, in that order. They have good basics in building settings, populating your universe, and general background knowledge.

My choices for dog meat are Navis Primer and Faith and Coin.

And for adventure books (you kids and your slang terms...what is a splat book?) I'd get the freebies, including the contest winners on the website.

All that said, there's not a book I don't find worth the money I paid for them, and I think I have all of them. I just find some get more use than others. But that would change with GM styles, too.

I think Into the Storm is almost a gimme for first slot. While I agree with Errant that the Alternate Careers are a bit off, (But then again, I don't really like any part of the career system) the rest of the book (Equipment, Vehicles, Port Wander, Social Rules are all worthwhile.

From there, it depends really as much on your group as anything else. If your group is 90% Away Missions, Battlefleet Koronus is a waste, but Stars of Inequity might be invaluable. So I'd see what you feel like your missing and get the relevant book, rather than sticking to a particular order.

I seem to recall one of FFG's staff mentioning that Into the Storm included all the stuff they wanted to put into the core rulebook but couldn't fit because it didn't have enough space. Agree that one's a gimmie for the first slot. Other good ones are Battlefleet Koronus (provided you ignore the abysmal ground combat rules), Koronus Bestiary, Edge of the Abyss, and Hostile Acquisitions. All the other books are decidedly "take it or leave it". The adventures are of varying quality - Lure and Soul Reaver are okay, if a bit railroad-y and have good background material that can be adapted as needed.

Cheers,

- V.