Bryan Caplan's Selfish Reasons to Have More Kids is a wonderfully gruntled book.
Based on twin and adoption studies, Caplan concludes that middle-class parents really don't need to overwhelm themselves and their children with a scheduled and directed childhood. In the long run, if you give your kids a vaguely normal childhood, they will turn out like you. The intense interventions have an effect in the short run, but tend to wash out in the long run.
Therefore, raising kids is easier than you probably thought. And kids are great fun for most parents. ERGO: have more than you were originally planning on.
The best advice he gives is that you should think long-term about your own parenthood. Sure, kids have lots of up-front costs in money, time, effort, and sleep, but the payoff later is huge. In fact, Caplan argues, you should pick the number of children you have based on the number of grandchildren you want to end up with.