Normally I read one book for entertainment (alternating between fiction and non-fiction) and one or more technical books, programming-related stuff. If I end up reading 3 books, it has to be one fiction, one non-fiction, plus the technical. But if that happens is probably because one of the books are boring, otherwise I will stick to one book.
I am a very slow reader, in part because I am born like this (I've tried to not mentally vocalize while I read but I find it impossible) and in part because English is not my native language, and pretty much all the books I read are in English. And then on top of that I rarely have time to read except when I am in bed, so it takes me only 20 pages or so to be defeated by sleep! So if I alternate too much between books, I feel I will never finish reading anything..