I'll throw my Imperial Guard helmet in the ring along with everybody else rooting for a book of locations and settings.
However, I would like to see an opening chapter covering the court of Governor Hax. This then followed by a chapter for each sub-sector, mentioning a sub-sector governor, notable NPCs in the sub-sector Governor's court, a rundown of important planets/stations within said sub-sector (again including important NPCs relating to the individual places, and also sub-sector encounters/happenings/phenomenon, not relating to the planets themselves, that have been reported in the past (e.g. Eldar pirates, Warp Storms, a Space Hulk flight path etc. etc.)
The closing chapters could be dedicated specifically to the operations of the Inquisition's separate factions within the sector, and stats for any Calixis Sector beasties of note (space-kraken, anybody?!)
There is loads of amazing material already written about the campaign setting, it's just a bit cumbersome juggling through so many reference books to find what you need. I think that a "Book of the Sector" would be a great way of tying all the brilliant strands of DH information together.
Just my 2ยข.
Well, I guess there is a market for this kind of stuff after all... I wasn't even sure to be interested in Creature Anathema (the monster-as-enemy concept in RPGs, so deeply xenophobic) but I will probably buy it anyway and there's a chance there's gonna be a second one. **** my completist side !