An excellent book which isn't specifically clicker but does look at the basics of learning theory is Emma Lethbridge's 'Knowing Your Horse' which is available from Amazon

It has a great mix of theoretical explanation and practical application (and some lovely neds too

). I would recommend it to anyone who has anything to do with horses as it goes through the different things that we do and explains how and why they work and the pros
Ben Hart is another one which is a good read (he also has a website if you do a google).
I would also recommend books by Karen Pryor, 'Don't Shoot the Dog' is a good one to start out with.
Of course you can also ask questions on the clicker board on here, quite a few of us use it and come at it from different angles (I don't use Alexandra Kurland so much but do use a bit of Ben's stuff and a lot of Emma's).