I resolved - partially - the problem of not exactly books, but photocopies of different scientific stuff, photocopies which overflow even more than proper books do. I got myself a scanner and although it takes longer (but you can do something else at the same time) I put them down in the electronic version. Saves lots of space.
And I don't mind too much reading from the screen.

Local library: your best friend (spurious remark, I know).

Bottom line: don't go anywhere near booksops, as Nineveh said. But stay also away from Amazon and Ebay. Stick to L_J :)