I highly recommend
Healthy Sleep Habits, Happy Child, by Marc Weissbluth. I love the importance this book places on sleep and that it backs this up with
research unlike many of the other books out there. It emphasizes the importance of having your child on a schedule around 4 months. Personally, I think a flexible sleep routine is beneficial before this time, but that's just my opinion.
Healthy Sleep Habits, Happy Child does suggests allowing your child to cry as long as necessary at night which I know some people will have a hard time with. BUT, it focuses on
perfect timing, among other things, to prevent crying at sleep times (see
Wake time) which is a great concept.