Besides the outstanding, if not "mandatory" 20th Century suggestions above, try these on for size (I still revisit them in my home library, often):

"The Camera Viewed: Writings on Twentieth-Century Photography", Vols 1 & 2 (Softbound), edited by Peninah R. Petruck.

"Photography in Print: Writings from 1816 to the Present", edited by Vicki Goldberg.

"Diana & Nikon", by Janet Malcolm.

"A World History of Photography", by Naomi Rosenblum.

"American Photography: A Critical History 1945 to the Present", by Jonathan Greene.

"Looking at Photographs", by John Szarkowski.

"Aperture Magazine Anthology: The Minor White Years, 1952-1972", edited by Peter Bunnell (recently published).

Essay: "The Work of Art in the Age of Mechanical Reproduction" by Walter Benjamin.

In addition, seek out every monograph (of interest).