Interesting books

Having been ill recently, I've not had a chance to get Richard a birthday present.  I'm thus looking for recommendations for interesting books.

Interesting in this context means chewy.  Coffee table books are out.  Recreational mathematics, physics, astonomy, earth science, natural history, biology, English history are all good subjects (but not necessarily the only ones).  The key thing is that books are in depth (think university level) and chock full of factual interesting material that is well written.

What have you read recently that would fit the bill?  (the book doesn't need to be in print - we're perfectly happy buying second-hand if the book is interesting)

I'm leaving comments visible as I'm hoping that people will bounce off each other's ideas.