Books you might want to look at are

Victor Burgin Thinking photography, Photography in print and Light Matters both by Vicky Goldberg, and Classic essays on photography Trachtenberg .
These are collections of essays by photographic theorists and photographers
An good intro into the theory is
David Bate Photography, and I would highly recommend Stephen Shore's The nature of photography, Criticizing photographs by Terry Barrett is an interesting insight into evaluating images , If you have an interest in Still life looking at the overlooked by Norman Bryson is worth reading. The classic theory of photography books are Barths camera Lucida and Susan Sontag on photography . Liz Wells comes from a direction which some might describe as marxist and has an academic style that can leave some people greatly underwhelmed .