It's a book with chapters. Basically a regular ass book. When kids are real little, their books are like 15 pages long. Then in like 1st or 2nd grade, they move onto reading big kid books - aka "chapter books" that have enough pages to warrant chapters.

You never hear someone over the age of 7 or 8 mention reading "chapter books" because they're just know as books.

Except anon, who is dumb as fuck.