This is not a "how to" book and it's difficult to read (it's philosophy), but if you want to absorb the theory and ideas that have shaped the modern understanding of photography buy it and read it.

This is a classic work in the philosophy of photography that will challenge your motivations and sense of purpose as a photographer. Note: Sontag is also covered in "The Photography Reader" listed above.

Salgado is one of the greatest living masters of photography, and many consider this his best and most passionate work. Salgado is controversial because so much of his work concerns the poor and powerless captured with such skill and grace. He demonstrates a level of commitment and focus that are essential to morally attempt this kind of work.

Josef Koudelka is one of my favorite photographers for his subjective immersion and documentary eye. This paperback has high quality reproductions of his work at a very low price, plus an essay and interview giving deeper insight into his work.

If you're not involved in conversations with other photographers then maybe it's time to start. Or, perhaps, try listening in on the conversation between the two who wrote this book.

A book by a photojournalist that traces her evolution from an "good enough" newspaper photographer into a more subjective and engaged documentary photographer. The book includes photos and texts explaining the images. The book spells out the changes a photographer goes through to bring her work to a more developed and mature level.