This I am not sure of. I believe a safe rough estimate would be 500. (Currently, anyway. Counting all the books I've ever owned, probably 900. I got rid of a lot of books when I was younger.)
2) The last book I bought?
3) The last book I read?
Cover to cover, Freakonomics. I read 3/4 of Linux Unwired this morning over a biscotti and Earl Grey, and the remaining 1/4 won't mean anything to me until I've got a cell plan or something with IR connections.)
4) Five books that mean a lot to me:
If you know these books, their significance is probably easy to determine. Here's another few for good measure: