Fairly often I put books in as loot in my games. Or offer up books as sources of knowledge of sought out.
Recently I have been reading a number of books about English history. Written by English writers. What is striking is how often they assume you are intimately familiar with modern England and its layout and geography. Also, that you are familiar with European power structures, religion, and noted people and books.
In a world with fewer but more widely read books, this will be magnified.
Now imagine the society for which that book was written is gone. The books it references off hand are lost or obscure.
This justifies a lot of vagueness. It explains a lot about why players can't expect a found book to clearly spell out what they need to know. The author always makes assumptions about the background of the reader. If the expected knowledge isn't there, you don't even need a code or deliberate vagueness for the information to be tricky to puzzle out.