I'm surprised no one has mentioned John Szarkowski - "The Photographer's Eye", "Mirrors and Windows", and especially "Looking At Photographs". Also check out any prefaces from the books published under his watch at MOMA. It's worth it to track down his essay from the OOP 4 vol. Atget retrospective, one of the most eloquent and insightful essays ever written on a photographer. On the flip side, Richard Bolton's "Contest of Meaning" was required reading during the initial MFA bubble in the late nineties. While interesting for its contrarian stance to the prevailing modernist, historical POV as laid down by the powers that be at MOMA, "Contest of Meaning" hasn't stood the test of time as gracefully as the humanist perspective that Szarkowski championed.

Bill Jay was a fun, insouciant critic of photography as well being a photographer himself. His portraits of other photographers are pretty great...http://www.billjayonphotography.com

I've never been able to understand how Sontag's lackluster work became required reading for undergraduate students in photography. My experience has been that students' work gets noticeably worse after they have read "On Photography".