Cookbooks and domestic books together, foreign language and other language geekery together, work-relevant books at work, a box of metalworking and machining books squirreled away in a box for when I can actually use them, and everything else wherever.
After a decade of averaging a move a year, I've managed to get pretty ruthless about not admitting onto my shelves non-reference books that I can get from a reasonable interlibrary loan system. Of course, as I'm sure you all will sympathize, a mess of somewhat esoteric interests still leaves me with <measure measure measure> 28 shelf-feet of books (at home).
-- Andy