I have built up a pretty good library over the last 20 years, but I am always drawn back to the first five that I was exposed to and then later bought. In no particular order: "The Americans"- Robert Frank, "Still Time"- Sally Mann, "Edward Weston: Seventy Photographs"- Ben Maddow, "Let Truth be the Prejudice"- Eugene Smith and "the Portfolios of Ansel Adams.
Currently as my budget and time allows, I am accumulating the early books by Ralph Gibson.
My most recent purchase off the used bookstore shelf is "Landscape: Theory", which includes work and essays by Robert Adams, Lewis Baltz, Brett Weston, Callahan, Paul Caponigro and George Tice among others. The reproduction of the work is poor, but the essays were well worth the $2.50 cost.