A very good starting point for anyone who wants to expand their understanding of photography. Starting from the ground up, Langford introduces his readers to the science, theory and practice of photography in clear, easy to understand terms without being condescending.
As someone who has always been interested in photography but, until recently, had never really taken the time to read deeply on the subject, this book was a great help in my progression from "beginner" to "serious beginner" and then "amateur". :P
It's useful for film and/or digital photographers, as much of the information applies to both formats, although some of the book deals with film alone.
I also recommend "The Focal Encyclopedia of Photography (4th Edition)" to anyone who wants to truly acquire a thorough knowledge of past, present and future photography.