The Rotary Swing, by Chuck Quinton.  This book is fantastic:  Simple, concise and easy to understand.  Chuck Quinton is probably golf teaching's best kept secret.  He has had a huge influence on my own game, and on what I teach on a daily basis through his excellent website RotarySwing.com.  He has taken Jim Hardy's concept of a One Plane swing,  powered by the body rather than the hands and arms, and simplified it to make it accessable to golfer of all ablilities.

Mastery by George Leonard. I dip into this book regularly, especially when things are not going so well.  It reminds me that improvement is a continuous process, rather than a finite objective.  It also reminds me that I'll never learn everything about this game, and that I need to always look at things from the point of view of the beginner, especially when I'm helping other people learn the game.  If you like this, 'The Way of Aikido' by the same author is also a fantastic book.

By Stan Utley. Putting is a very personal thing. I like to keep things as simple as possible and this book is very straightforward.   Stan Utley teaches a traditional method where the putter face stays square to the swing path, rather than the target line, with the stroke moving in an arc. I find this a much more natural way to swing the putter and my putting has been getting more and more consistent as I have got used to this stroke.