1) Total number of books owned?
About 4000 science fiction, about 200 computer, about 1000 other. (Plus about another 2000 in the house belonging to my wife, and another few hundred belonging to my kids...)
2) The last book I bought?
Peter Hamilton - Pandora's Star. (Well, MJD's Higher Order Perl was ordered earlier, shipped about the same time, and received later, so it sort of dependent upon the precise definition of "bought".)
3) The last book I read?
Higher Order Perl.
4) Five books that mean a lot to me (for sufficiently large values of five):
The Mythical Man Month - Fred Brooks
Programming Perls - Jon Bently
The Art of Computer Programming - Donald Knuth (lots more)
Leslie Charteris (the Saint)
Erle Stanley Gardner (Perry Mason, Donald Lam, Doug Selby)
Gregory McDonald (Fletch, Flynn)
G. K. Chesterton (Father Brown)
Agatha Christie (many)
Godel, Escher, and Back - Douglas Hofstadter
Surely You're Joking, Mr Feynman - Richard Feynman
Patrick O'Brian (Jack Aubrey/Stephen Maturin stories)
Bridge in the Menagerie - Victor Mollo
(I would probably come up with a totally different list every time I was asked.)