I have found the David Leadbetter books quite useful. HID got me the "Faults and Fixes" and "Positive Practice" for Christmas which are quite simplistic but help hugely. They aren't overly technical and the illustrations are rudimentary but they show you clearly and concisely what he's on about
It really depends on what stage your game is at and what time you are willing to put in.I would recommend the following 4 books.
1 Nick Faldo- A Swing For Life
2 Dr Ralph Mann- Swing like a Pro
3 David Leadbetter- 100% Golf
4 David Leadbetter- Positive Practice
Avoid Hogan's 5 Lessons The Modern Fundamentals of Golf as Leadbetter's The Fundamentals of Hogan is a better read. Tiger Woods- How I play Golf is okay especially for the short game, but you would have to get it cheap.
I could recommend another few but I think the top four are really all you need.