In mathematics, there are whole books almost completely dedicated to problems you are supposed to work out. Or the prose is terse and dense, like Rudin's textbooks on real analysis. They are valuable books, but they aren't often recommended to beginners. You can use a bit of sugar to make the medicine go down.

I think books are often long to give the reader some space to think. Perhaps the best readers know how to create their own space, but the worst readers require the sugar. In other words, books are long because people aren't good readers.