Common in some older books, and modern books in certain fields. Some (philosophy is where I've see this quite a bit) will instead put from a paragraph up to a c

Common in some older books, and modern books in certain fields. Some (philosophy is where I've see this quite a bit) will instead put from a paragraph up to a couple pages describing the topics a chapter/"book" covers at the beginning of the chapter, which allows for a little more space than cramming it into a TOC. Helps with "OK I know that was in here somewhere but which chapter was it..."