I think I am a pretty easy-going person, but one thing that really gets me irritated is when people who don't have kids, or people new to the realm of parenthood decide to enlighten you with their child-rearing ideology.
If you don't have any kids, then my opinion is that you have no validation by giving parents advice at all. Unless you are a teacher. Your best bet is to just look at your friend or colleague and say, "I know nothing on this subject, other than what I've read, and I'm glad it isn't me." It's kind of like men weighing in on abortion.
If you have a baby, I feel worse for you, giving me parenting advice, because it shows you are still under the assumption you will have some control over this child when it becomes old enough to make decisions on its own. Either you will continue to live in a fantasy world, or you will get informed real fast, the first time your sweet child says that they hate you.
While having one child is definitely a learning experience, I feel like it is the junior varsity level of parenting. You don't make the senior squad until you have two children who now introduce the art of mortal combat with one another. This is when you begin to loathe the notion that having kids isn't as hard as people say it is, because anything you learned before becomes obsolete.
I also have to laugh at parents who are anti-germ, hand sanitizing fools. You are fighting an endless battle, and your kid may get sick more often because of it. A little dirt never hurt anyone, and the kid needs to build an immune system.
In the end, books on parenting are sometimes helpful, but the best place for guidance is probably your friends and family who have gone through it before. Books present a best case scenario, and much like the media, present situations in extreme opposition. Think back to when you were their age, and how much control your parents really had over you. Don't be dumb enough to think that you are smarter than your parents were. Kids are our karma coming due for all that we did and managed to get away with.
The sooner you accept that, the less you will be surprised by.