Every single new one that I find and then because there's so many, I'll reread one I think is new before looking at my checklist and seeing that I already read it.
Spoiler
When first cracking open one of the Black Books on my own it only took me a couple of seconds to recognize what plane of existence I was on despite having never been there before. If I was in Apocrypha for reals, it would have taken me an hour to get to the next room because I'd be sitting on platform you materialize on, reading all of the scraps of paper on the ground and whirling around in the air before moving on. Hermy Mora would have my soul in seconds.