After you've purchased the "essential" books for the game (for me, that means rulebook, GM's screen), I would suggest taking a good, long look at this item.
As described, it gives you info on all of the Houses' planetary holdings; the planet's history, current situation, exports, etc. You also get names of notable personsages which populate their respective planets. All in all, one gets a pretty darn good amount of stuff by which to create games by.
Oh, and you also get listings of Imperial holdings as well as d20 info for animals, classes, and so on.
The down side to this work, if you could call it that, is that it does not cover the non House and Imperial planets (so perhaps the title of the book is a bit off), but I suppose Holistic couldn't just give out everything at one time.
My opinion: A pretty good buy.