I second Roland Barthes' "Camera Lucida" -- probably his most accessible writing. Much of his works on semiology, however, just gave me a headache... were I to attempt to go through them today, the exercise would have a devastating effect on my marriage -- both in and out of the bedroom!
"Diana & Nikon", by Janet Malcolm. (Elegantly written essays on photography and culture -- easier to digest than Sontag).
"The Work of Art in the Age of Mechanical Reproduction" (essay), by Walter Benjamin (I come back to this every few years).
"How to See", by George Nelson
"Art and Illusion", by Rudolf Arnheim
"Design and Form: The Basic Course at the Bauhaus", by Johannes Itten
"If You Want To Write", by Brenda Ueland (Not about photography or image-making, per se, but more on the creative process and its life-long nurturing).
"Concerning the Spiritual in Art", by Wassily Kandinsky
"Paul Klee on Modern Art", by Paul Klee
"Picasso: A Life"; vols. 1, 2 and 3, by John Richardson
"Photography & Fascination", by Max Kozloff
"The Americans", by Robert Frank
"American Photographs", by Walker Evans
"Callahan", by John Szarkowski
"Emmet Gowin: Photographs"
"The World On My Doorstep", by Paul Strand
"Evelyn Hofer", edited by Susanne Breidenbach
"August Sander: Seeing, Observing and Thinking", published by Schirmer/Mosel
"Thomas Struth: Portraits". (I have the closest affinity to him of all the Dusseldorf School/Becher photographers).