While this book (like the whole series) is outdated in some ways, having been published in the late 70s, I still found it useful.
Each book in the series has similar sections, including: a broad overview of the characteristics of that age, your child and others, routines and tensional outlets, discipline, abilities, school, and Q&A from real life. They are careful to point out that the information is for the "average" child, that your child may differ, and not to worry about that. They studied thousands of kids to find out in general, what happens developmentally each year.
There are a few things these books are not. They are not a collection ofconcrete parenting tips or a parenting manual. They are not designed to answer every question about your specific child's behavior. Instead, they provide overviews of what you can expect in behaviors, emotions, and skills. Is your 5-year-old so positive that you're waiting for the other shoe to drop? Did your sunny 5-year-old become inexplicably defiant or explosive around the half-year mark? Completely normal. I find these books nice because they give you a view into what's going on with your child and what might be round the corner. My son just turned 5 and because I know that children develop at different speeds, I read the 4-year-old, ,5-year-old, and 6-year old books. I found them to be pretty spot on in terms of what I'm seeing. Boy, reading the 6-year-old book made me a little anxious and all the more intent on enjoying 5!
I find the discipline section the least useful because although they were definitely ahead of their time on positive discipline, we have come a lot further in the 30 years since the books were published. I read it because I'm curious to see what my parents were being told about parenting, but I don't take it to heart.