So today I went to my University office (for the first time in months) and spent six hours triaging books: discard, leave with the department, or take home. Somewhat to my surprise, the three categories turned out almost exactly equal in size.
This exercise entailed throwing out a lot of proceedings for theoretical-CS conferences I attended and was very interested in at the time, but I realistically haven't done any TCS research in fifteen years. And if I did somehow get back into TCS, it would be easier to find the papers on-line than in a printed volume anyway. But throwing out the proceedings (except the few in which I had papers) carries an air of finality.
My research for the past fifteen years has been mostly in CS Education, so I had three shelves of proceedings from CSE conferences. I kept the ones in which I had papers, and left the rest to the department, on the theory that somebody on the CS faculty will be interested in them. Again, leaving this stuff is a final acknowledgment that (a) my research in CSE will be limited if I'm not in a classroom regularly, and (b) a lot of this stuff is on-line anyway.
I threw out most textbooks older than 5-10 years, except a couple of "classics" and those of which I have particularly fond memories. I left recent textbooks to the department, as above.
Still, I have five good-sized boxes of books in the car for which I haven't found homes in the house yet.