About one per month is also my average as well. So far, these are probably the ones I don't think I'd ever want to part with:

1) Stay This Moment by Sam Abell (2 copies - one still new & shrinkwrapped)
2) Europeans by Cartier-Bresson
3) Perfect Times, Perfect Places by Robert Adams
4) Cape Light by Joel Meyerowitz
5) Alfred Stieglitz at Lake George, edited by John Szarkowski

Of these, only Stay This Moment has any real value (the HCB book is a 90s reprint), but, if I may use a cliche, these books speak to me more than everything else I've come across in my ten years of book buying.

It pains me to leave out the book I read most often: Examples - The Making of 40 Photographs by Lord Ansel, but it is more of a technical book than one that introduced art to the world.

Finally, I buy most new books at Barnes & Noble or the International Center of Photography's bookstore, and most used from ebay or Amazon marketplace sellers.