This is a fairly good introduction to photographic theory… From a technical standpoint, Michael Freeman ‘s Textbooks are quite informative… I’ve only read (most of) The Photographer’s Eye, and it has given me a lot of useful tools to use while out shooting. So few photographers understand that the art is more than an empirical record of an event. It takes the event beyond its historical context and brings it into the present, long after it’s over. I could go on. Or, you could check out the link and the listed reading material.