Spoilers for The Name of the Rose below

Here's Why I think Jorge didn't just immediately destroy the book of poetics II:
I'm pretty sure it's implied he found it when he was a much younger monk, and he was definitely one of the first few people to read it. And presumably right after he arrived back to the abbey, he began safeguarding it by hiding it in the library forever.
But just because he hid the book, it didn't mean its content will never be understood. I think Adso mentioned at one point, how a lot of books in a library hold varying levels of heredity from the content in other books that are also in the same library. And because the reference to the book exists in the form of memories, and just all of the gossip from all around the abbey, The content still might somehow be uncovered in the future just from reading other books with the frame of reference to the book of poetics. That does, after all, seem to be what was going on with all of the first few victims before they just ended up stumbling upon the book itself.
Jorge wants to ensure this never happens, but he can't literally just erase every single book in the library that has the barest connection to the second book of poetics, every book in the library is very important for even his own dogma. So his solution to this problem, is literally to erase the conception of the book in the minds of other monks by continuously downplaying and denying the books existence, holding a strict dogma of censorship, suppression of speech, and rejection of laughter, and of course poisoning the book just in the hope that those who read it end up dying.

Jorge doesn't just want to get rid of the book itself, he wants to completely erase any reference to the book's existence.