There could be lots of topics on a book, such as the form/style, the author. In contrast to which, there could also be another topic, namely, the content.
An answered to a post (https://qr.ae/pX80PJ) says
"contents" is grammatically correct but only if used for the things that are inside a box, bag, room etc OR the things that are written in a book, letter etc. This will be considered as plural, countable.
where she uses "contents" to refer to the things that are written in a book, I suppose "things" is interchangeable to "ideas".
Another post gives this explanation
The singular form content can only be used to refer to something abstract or not easily dividable into parts.
Nevertheless, Cambridge Dictionary clearly defines the word "content" as [ S or U ] with the meaning "the ideas that are contained in a piece of writing, a speech, or a film"
I guess the part about book in that quote is incorrect. Is my understanding right?