If you're meaning the tie-in novels for doctors 9 and 10, there's 13 of them out so far, with another one published in March 2008 (odd because they're normally out 3 at a time), and another 3 in April 2008...
Not to mention all the other tie-in books and stuff the BBC has produced (like complete guides to individual characters/races - oh, and the new Doctor Who Encyclopedia is AMAZING...)
And don't forget the tons of books for the original series (I highly recommend the 'Short Trips' collections if you can get them!), which are mostly out of print now...
For the full low-down see
http://www.waterstones.com/waterstonesweb/simpleSearch.do?simpleSearchString=doctor+who&searchType=0&Image1.x=14&Image1.y=10 (you can sort it by date too if you wanna see what's coming up...)
Oh, and I dunno if there's USA rights for a lot of them yet, so you can order from there and get it posted if there's not - I can find that out for you on Saturday though (yes I'm a bookseller... anything you need to know about the books, I'm your lady!
xd )
ps - if anyone wants, I can list the titles of the available novels if anybody wants to get them?