By far one of the most generative and enjoyable things you can do for yourself and your work is to feed your mind well. The following books and other materials are all materials that I have personally found value in. Most will relate directly to photography in some aspect, but not all of them will. Those that aren't directly related to photography will be of value in some other way that supports photography, such as those books relating to creative process. The following are presented in no particular order.
The Education of a Photographer (Charles H Traub, Et Al)
Edited by Charles H Traub, et al, this is an enriching collection of essays by a range of photographers spanning the twentieth century and into the early twenty-first. It is important to look at the work other photographers have produced, as there is an immense amount to be learned that way, but for many it's just as important to read the writings of other photographers. Essays are windows into the origins of the images, and understanding more about the internal creative spaces of artists better allows us to understand their work and processes as well as our own. Worth reading and reading again.
Thoughts on Landscape (Frank Gohlke)
All coming from the mind of photographer Frank Gohlke, this collection of essays and interviews gives a fascinating look into the inspirations and origins of one of the ten photographers who were part of the influential 1975 exhibit New Topographics at the International Museum of Photography. Gohlke's ideas are clear and presented in an engaging voice that gives great insight into his work.
Steal Like an Artist (Austin Kleon)
This is one of my favorite books relating to the creative habit. It's not terribly long, at about 140 pages, so it's not an intimidating read. The text is written with life to it and it's full of practical wisdom about being a creative entity in the twenty-first century. Many of us are constantly getting in our own way and feel frustrated with our lack of progress. This is a good tool for helping clear the way and building forward momentum, whatever our form of creative expression.
The Artist's Way (Julia Cameron)
Julia Cameron's classic The Artist's Way is a juggernaut of creative personal development books. In the form of a 12-week course, it is a program aimed at listening to your inner Self more effectively and learning how to better support and engage in your creative practice. If you find yourself with an icky feeling about the A Spiritual Path to Higher Creativity subtitle, get over it. This book changed my creative practice permanently and for the better and it has done the same for many, many other people. It isn't for everyone, but if you find yourself struggling in your process, consider giving this book a chance.
Disclaimer of sorts: The Amazon links are affiliate links. This means you pay the same price for the thing you buy, but I get a small cut that helps me defray the costs of the web site and such. Apparently it's a good idea to make this known (for legal reasons). Purchases made via these links are sincerely appreciated.