I'd say there's not really any surprises here. The only books that I get a lot of use out of are the core books. After that, probably PsiHB, some of the splats (T&B, S&S), and a little bit of Fiend Folio and Mindscapes (although the use is pretty much restricted to the one page with the revised psion powers chart). Nearly everything else gets one read through and just an occasional reference. If I was starting a new campaign now, I'd probably add Book of the Righteous to the list but I'm not using it much now.