I'm translating a literature list, and roughly translated, one of the entries reads "Any book of approximately 250 pages, dealing with the topic chosen for the argumentative essay" – but the word corresponding to "any" has a much stronger sense of 'chosen by the student'. If I were to rephrase it, I'd say "The student should choose any book of approximately 250 pages, dealing with..."
My question, then, is: is there a single adjective that I can use here, in this sense?
I've thought about "An optional book of...", but that seems to imply that the student can choose not to use a book, rather than that they are supposed to use a book, but that they may choose any book. I've also thought about "A choice book of..." but I have a feeling that that means 'well-chosen' rather than 'possible to choose'?
As always, I'd be immensely grateful for any help I can get :)