Who hasn't heard of Nancy Drew and the Hardy Boys? They are perhaps the most successful series books ever in terms of longevity, and they changed the face of series books for decades, so that the detective story dominated. But in the early twentieth century, many other kinds of juvenile series books were popular, and the themes reveal something about the culture of the time.
Many early series were devoted to new technologies such as movies, motorcycles, cars, and airships. Flying stories were especially popular during World Wars I and II. Scouting and other outdoor adventure stories were quite popular until the advent of the detective story. Sports series survived, however, especially those about basketball, football, and baseball. Perhaps the best of the lot, the Chip Hilton series, has been recently reprinted.
The older stories are especially intriging because they reveal little things about the time they were written. For instance, in the original Nancy Drew stories from the 1930s, Nancy has to keep an eye on the weather if she's going to be driving anywhere. Why? Not many roads were paved then, and the roads would become impassable in the rain. Running boards on cars show up frequently, and in the Rambler Club a friendly official calls speeding "scorching".
Classic series books references may be seen in Hollywood and even in modern vocabulary. For example, the word taser stands for "Thomas A. Swift's Electric Rifle." (The book Tom Swift and His Electric Rifle was published a hundred years ago.) In the original That Darn Cat, the supporting actress mentions "Tom Swift and His Electric Switchboard". The Rover Boys are a frequent reference in older movies.
It would be interesting to know just how much series books influenced those who read them. But if only a reflection of their time, the classic series book is certainly a significant part of Western history and culture.