I believe "liber de bellō est" would mean "the book is about/concerning war". What question would elicet that as a response? I.e. how would I say "what is the book about/concerning?"? "De quid/quo est liber?" Or would I need to use māteria or rēs to say something like "what is the subject of the book?"
Gratias vobis ago Paulus
A natural phrase to mean to "deal with"/"a question of" is de aliqua re agitur. We can thus have in hoc libero de arte belli agitur (This book deals with the art of war). There are indeed several examples (non classical - for now did not find a classical one specifically about book/works) like: LIBER SECUNDUS In quo agitur de Processu causarum Appellationum. The question to elicit this would therefore be de quo [negotio] agitur in hoc libro?.
I chose to first adhere to the common expression with the passive agitur; however, it might be more fitting to simply use the active ago. We have the phrase agere de/cum aliqua re, while extended in scope, can be used in our case. Hence we could have the question: cum/de qua [re] liber agit?
There are more options, namely other verbs that can be used instead of the monstrous ago: One of them is the verb tracto which can be attached directly with de. Again, I have found many non-classical examples specifically about liber.
