The term "chapter book" doesn't mean much, evidently. Yesterday at Seven Impossible Things Before Breakfast Jules and I discussed the lack of agreement about terminology to describe books for new readers. "Chapter book" is only one name used to label shorter books with limited text, somewhat easier vocabulary, and maybe some illustrations written and marketed for children, say, 7 to 9 years old.
Unless, you're The New York Times. Scroll down its Children's Bestseller List until you get to the subtitle "Chapter Books" and check out the age designations after each title. Every book there is either middle grade or YA. It appears that The NYTimes considers the term chapter book to mean "a book with chapters."
To find books for kids under eight- or nine-years-old, at least on this week's list, you have to look under "Series Books" where Junie B. Jones, Fancy Nancy, and The Magic Tree House are listed. They are series books, but isn't Diary of a Wimpy Kid, too? There are two Wimpy Kid books under "Chapter Books."
My regular readers know that I crave order and definition.