Jon's canon has books that are usually referred to as 'Leitners', named for a gentleman named Jurgen Leitner who attempted to collect and catalog as many of them as possible before his library was eventually dismantled. The Leitners are totally normal-looking books that are actually 'pure' expressions of the otherworldly Entities from his world; reading them with intent lets you use their effects, but reading them WITHOUT intent (or sometimes, at all) can lead to the book, well... reading you.

With his arrival, are there Leitners in the library? Are there perhaps defanged Leitners in the library (aka, you can read them without any effects happening)?

Jon's going to act under the assumption that they're there (they tend not to show up in catalogs or organizational system just kind of jammed between things so even if he can't see them in a search or an index, he won't believe it) but I'm curious for possible fun plot stuff down the line if it was possible.