In discussion following my presentation to the ACL 2 seminar, Bob Boyer said "Shoenfield is the bible." And while talking with Elaine Rich before my Dean's Scholars lunch presentation, she strongly recommended Brachman's KR book.
This brought John Udell's library lookup gizmo out of the someday pile for a try. Support for the libraries near me didn't work out of the box. It's reasonably clear how to get it working, but a manual search showed that the libraries don't have the books anyway.
In the research library symposium I learned that even some on-campus researchers find it easier to buy books thru Amazon than to use their campus library. I have no trouble adding these two books to my amazon wishlist, but I hesitate to actually order them.
I make time for novels by McMurtry and Crichton and such, but the computer book industry is full of stuff that is rushed to market. If there's a topic that I'm really interested in, I can download the source the day it's released or follow the mailing list archives and weblogs pretty directly. By the time it has come out in book form, the chance to influence the direction has probably passed. So when I consider having my employer buy a book for my bookshelf, I have a hard time justifying more than $20 or so.
I devote one shelf to books that were part of my education... starting with The Complete Book of Model Car Building and A Field Guide to Rocks and Minerals thru The Facts for the Color Computer, OS/9, GoedelEscherBach, SmallTalk-80 and so on thru POSIX 1003.1. You might try the MyLowestBookshelf excercise. I had some fun with it a few years ago.
I do keep one shelf of Web and XML and networking books; not so much so that I can refer to their contents but rather to commemorate working with the people who wrote them.
I have Lessig's Free Culture on the top shelf, i.e. the "you really should have read this by now" guilt-pile. But I had better luck making time to listen to the recording of his Wikimania talk over the web.
For current events and new developments, I'll probably stick with online sources. But I think I'll order these two books; they seem to have stuff I can't get online.
postscript: for an extra $13.99, amazon offers to let me start reading the Brachman right now with their Amazon Online Reader. Hmm...