So I'm not planning on running any other game besides rogue trader. I have almost all the RT books and resources (and will have absolutely everything by christmas). What I'm looking for are other 40k rpg books such as those from DH, DE, OW and BC that are considered awesome resources that are useful to RT so I can look into them while they can still be found. I particularly like setting descriptions like Port Wander from Into the Storm or troves of sample NPC descriptions and other such fluff. I think the RT line has enough in the way of class/skill/race choices, so I'm not looking for that sort of thing really. Thanks in advance!

Mostly, I'd say it all depends on what sort of Rogue Trader games you run, and what sort of Rogue Trader your players portray. Personally, I'm a fan of several Deathwatch books, if not so much the Deathwatch, as it is portrayed. If you like the idea of throwing Tyranids, Tau, or Necrons at them, the DW core book, and Mark of the Xenos are really good, and the Necrons get covered in The Outer Reach. Especially if you want your RT game to feel more militant, you could run it in the Jericho Reach, where an endless, manifold war is in full swing. For this, these Deathwatch books are ideal. On the other lines, Only War CAN be good, if you plan to either tote around your own regiment of troops, as some Rogue traders can do, or if you plan to go rogue, and have to expect to fight them, instead. Black Crusade CAN be good if you are planning to fight, or join, Chaos, and DH2 CAN be good if you want to meddle in the affairs of the Inquisition, but all three of these use a different mechanical system, to a greater degree, so considerably more doctoring/converting might result, even if you do believe it's a better system than DH1/RT/DW uses. As well, a lot of them contain stuff that RT already covers, to some extent.

Were it me, I'd grab DW core, and Mark of the Xenos, to round out your militant options for enemies, and consider The Outer Reach, and Rites of Battle, for Necrons, if you actually like that idea, and some really good vehicle options, if you decide your Rogue Trader can, somehow, acquire Astartes-level rides, or pinch some from the Tau. Do note, though, that the enemies mentioned are often calibrated to fight a squad of Space Marines, so a squad of Rogue Trader and Co. might need to bring some troops, and/or their best gear, to round them out, and possibly more freedom to choose/manipulate their fields of battle, or they might get eaten. Still, few people can prep like a Rogue Trader, if given fair warning to do so.

I'd work on getting at least the DH2 core rulebook just to see the latest iteration of the FFG ruleset.

DH1 's Disciples of the Dark Gods is a great read- tons of handy plot hooks.

Awesome advice. I was actually looking at the DW stuff, so that helps.