If you're happy with how you were raised, felt loved by your parents and continue, if they're alive, to have happy relationships with them, then how lovely, you can trust your instincts and don't need to trouble yourself with a single parenting book. Good for you.
Some of us aren't. Some of us had deeply unhappy childhoods raised by parents who had a horrible time when they were kids. These books are lifesavers for us. And some people do get a bit evangelical and may come across as smug - it's probably borne of insecurity. They're doing their best.
How smug you sound, who imply i'm a credulous fool for not 'simply' following my instincts (instincts which are violent, shouty, irritable, short tempered, given to use shaming, easily triggered by normal child behaviour).
I'm an UP. I am sometimes ineffectual, though I'm getting more effective as I gain more experience of being a parent. I'm doing my best, same as most people. So find what compassion you can for other parents, or fuck off.