It may not be new or earth shattering info about the 90 minute cycles, but this book somehow made me understand them better, more clearly than other books. Yes, it does suggest letting your babies cry but you don't have to do that to get some benefit from the book.
Like pp's have said, it really helped me figure out when to offer naps. And I probably read it somewhere around the time ds wad 9-10mo. Basically the deal is there's a window of opportunity for sleep to come quickly every 90 minutes or in multiples of that. The author suggests, and I found it to be true, that if you offer sleep toward the end of that 90 minute(or 3, 4 1/2, 6 hour) period, the child will likely fall asleep quickly. Past that window of opportunity, it will likely take until the next 90 minute increment has passed. I was finding it was starting to take 45 min- an hour for my ds to
fall asleep. This was unusual for him and frustrating for me, so I saw this book and bought it. It was like the missing piece of the puzzle for me. I knew the signs of my sleepy baby, but I didn't get how to predict when they would
After noticing the time he woke up and figuring put when those 90 minute increments were, I was able to offer naps and bedtime when he would be most likely to fall asleep and it was really helpful. Instead of nursing for an hour before he'd fall asleep, (and feeling frustrated) we were able to nurse briefly and he'd fall asleep within about 10 minutes. So much easier!
Bottom line, I thought it was a really useful book and I just ignored the parts that I didn't like- crying, etc. I just wanted some info about how to predict sleep cycles because I felt like I was missing something and it gave me that.