Another book that you could try is Dr. Marc Weissbluth's "Healthy Sleep Habits, Happy Child". He is an actual pediatrician that practices in Chicago, and specializes in infant/child sleep as he has conducted over 30 years of research in this area. Yes, he does recommend CIO (both extinction and gradual), and goes into detail about this procedure. If CIO isn't something you want to do - no worries. Only you know your family best and the decision should be up to you!
I've read the book and started instituting the techniques at around 6 months. We, too, had great luck with night-time sleeping but had issues with the napping. And, just like you, our baby would do catnaps and we would see a difference in his temperament as well. My only wish is that I would have tried the techniques earlier. I was VERY consistent with it, and within THREE days our baby was on a great nap schedule (with flexibility of course), taking a 90 minute nap in the AM and at least a 2 hour nap in the PM. He goes to bed at 6:30 PM and wakes up between 6:00 - 7:00 AM each day. I expected hours and hours of crying. The longest span I got was 45 minutes. Everything after that was 10 or 5 minutes and had basically no crying after 1 week. It was easy because I learned how to pick up on his sleep cues and respect his internal sleep clock.
Now, at nearly 12 months, when it is nap time we just scoop up the baby and put him in his crib wide awake. He might play for 5 minutes or just sit there and zone, but he falls asleep on his own without fuss (I'm still in shock, after hearing everyone else's horror stories!).
Feel free to PM me with questions if you'd like, especially if you don't want to plow through the book. You could probably start sleep training around 4 months, so it would be right around the corner. We 'train' our babies to breastfeed (very few just latch on and go for it), we 'train' our babies to use a spoon, we 'train' our children to read and write, so it makes sense to help 'train' our babies to sleep too. You are the parent and you have seen what both effective and ineffective sleep does to your baby.