The WFP one is good and Chris Bartle's book is excellent and very thorough. There are lots of options!
What are you wanting it for? For a general overview of the sport or xc riding specifically? There are a few very good heavily illustrated xc ones - Blyth Tait's sticks in my mind but there are a couple of others.
Thanks all, particularly interested in cross country riding eg. how to ride different types of fences and how to assess the course to work out how you are going to ride it.
Obviously I will have some lessons too but I like to know the theory behind the instruction and I'm a bit of a book worm.