Learn French: Dans ce livre, il s'agit de la guerre.

The expression il s'agit de is impersonal, meaning its subject is always the neutral il (it), much like il pleut (it is raining). You cannot say Ce livre s'agit de. Instead, you have to establish the context first with Dans ce livre (In this book), and then use the impersonal expression il s'agit de (it is about).

What does s'agit mean on its own?

It comes from the reflexive verb s'agir, but this verb only exists in the fixed expression il s'agit de (it is about / it is a matter of). It is related to the regular verb agir (to act), so you can think of the literal translation as "it acts itself of".

Could I say Ce livre parle de la guerre instead?

Yes, absolutely! Ce livre parle de (This book talks about) is a very common and natural alternative. The expression il s'agit de is slightly more formal and often used to summarize the main theme of a story, film, or text.

Why is it de la guerre and not just de guerre?