P.G. Wodehouse is worth keeping at any cost. In case you aren't aware of it the Overlook Press is reprinting all of the master's works in batchs of three or four volumes every few months. They started last year and now have eight volumes out. The books are printed on acid-free paper and are wonderful to hold, read, behold, etc. The nice folks at Overlook will put you on an e-mail list to be notified when new books come out. You have to call them about that though.
Beyond Wodehouse my "must keep" books include the O'Reilly' books, anything by TheDamian, and a few other odds and ends about C++ and C. Of course, this doesn't include my collections of military history, railroad, and sci-fi books. That list gets a lot longer.
Just my $.02,
Ron