***note: anything that I discuss about this series will focus on only books 1-19, as they are the only ones written my Warner, and do differ from books 20 and up***
This is a series of books about four siblings in the 1940s and 50s. In the first book, they move into an abandoned boxcar to avoid being forced to live with their grandfather who they believe is a mean old man. They later find out that he is not what they thought, and they move in with him. With his encouragement and help they have many adventures and solve many mysteries.
This series originally belonged to my younger sister, but we were on a 20 hour car ride from Michigan to Florida as kids, and I was bored and picked up the first book, by the time she finished reading one of the books I had read through 3 and I was hooked. After that, my mother bought the series for me instead.
I've read some reviews that say this series may not hold for modern young people, but really, I read it in the mid-late 80s and I enjoyed it, and I think there is still plenty of entertainment value for today's kids. Maybe some kids will be deterred by the lack of technology, but if they enjoy a good adventure, I'm not sure they'll miss it. After all, the adventures that the children go on are often the kinds of adventures that many kids dream of (like spending the summer all by themselves on an island, living off the land) and the mysteries that they solve are often the type that only kids see (like a hidden room "no one" new existed).
One thing I really enjoyed, as I read from one book to the next is that you slowly so the characters growing up before your eyes. Each book took place months apart, so over the course of the series they age and grow. By the end you start to get hints of their future (like who one of the girls may end up marrying). This is a large reason I do not recommend reading the books 20 and up, as they take the characters back to the age that there were in book one. I didn't enjoy that aspect, and didn't think the stories were as good, either.
A few years ago, I was feeling nostalgic and decided that I wanted to read the books again. Obviously, it took me virtually no time to get through the series, but I actually enjoyed it as much as I did as a kid, and I actually found things in it this time that I didn't appreciate as a kid (history types of things and things that they could do in the 40s and 50s that wouldn't be allowed today). While some adults probably wouldn't enjoy this if there was no nostalgia for them, others mights find it a fun, quick read.