The first book covers the first two seasons, whilst the second book covers the next two (it's not hard maths!) Whilst the first book is a solid guide in itself (offering interviews with cast and behind the scenes folk, as well as episode synopses and little trivia tidbits), the second book is the real star here, by going into all of the previously mentioned in a completely exhaustive way, whilst also noting how continuing storylines develop and relate to past episodes. Especially concerning episodes such as 'Restless' (the excellent season 4 finale), the guides serve as a great way of enhancing the viewing experience by explaining things that you might not have noticed the first time, or didn't understand.
I really cannot hype these books enough - they're great to go back to again and again. Go buy! Now!