"Thin" is suggested in the drop-downs for צר, but marked incorrect if used.
Is there a reason for that ? "narrow" (the recommended term) is not one that I can really make much sense of, as an English speaker, as a property of a book.
A "narrow gate" makes sense, but a 'narrow book' ?
'Small minded' or something ?
I would use ספר דק for a "thin book". The sentence seems strange to me, my first reaction is to imagine a book that is sort of elongated, an unusual shape. And the opposite of that would be רחב. So this sentence does indeed mean "thick or narrow", although why would anybody phrase this question is unclear.
Your question made me consider the option that the book is sort of narrow in scope. Not so much small minded, but maybe covers a very specific field while ignoring relevant information that is not exactly within the scope... it could mean that, but it takes an effort.