"contents" or "content", which to use when talking about a book?

Question

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?

Answer 1

Both "contents" and "content" can be used to refer to things in a book, etc, but they mean slightly different things:

"contents" means separable pieces that are put together to make up the book. This may include chapters, appendices, figures, references, etc. These are often the things listed in a "table of contents" at the beginning of a book. The countable form "contents" is used here because they are separable pieces, which can therefore be counted as individual items.

(Note, "contents" doesn't usually refer to ideas, but rather to structural/organizational parts of the book (chapters, etc))

"content" means the representation of the ideas contained in the book, or the overall text of the work, taken as a whole. It is uncountable because it is not separable into individual pieces (without changing the nature of the pieces), and only makes sense as a single whole.

So you might say:

The book's contents included two chapters about penguins.

but

The book's content was mostly about motorcycle maintenance (despite some references to penguins)