I left a mountain of books behind when I moved - each time - then promptly started gathering books again, just not the same.
I kept all the technical books but I must admit that I should have not bothered - they age. There's a few I would have kept anyway, because they are so good, but I have so many and I still have them, since I feel guilty to just throw them away (anyone wants java 2 books?)
I now tend to buy my technical books as ebooks, at least they dont use up space and I know I wont need more than 1% of them in 10 years....
I do miss many of the french books I left behind, but I was moving to england and what good are books you cannot share or lend with your local friends. So I kept only few books in french...
I have too many books I wont reread that I should give away, if someone wants to come dig