I have to say videos have been more helpful than books - many available at cost but worth the money.
Since you asked about books, there are a few Kindle offerings;
Sporting Clays - Shooting for Beginners and Experts - by John King
Probably starts more arguments than it resolves - but of interest none the less.
Mind vs Target - by Bob Palmer
The mental part of the game - which I understand is major - but reminds me of my climbing instructor telling me to "be one with the rock".
Shoot Where You Look - by Leon Measures
I'm skeptical - but kind of a neat read.
Lots of others that I haven't looked at. Trying to talk myself into "Breaking Clays", by Chris Batha - but cringing at 29.59 for an ebook.
All nice in the off time, but none even nearly worth an hour with a good instructor.
Rob!