I have released 6 books at PokeStops. Sine we were out walking the area till an egg hatched, I was able to go back by the area I dropped the books off. All books were picked up, one was journaled the same day.
If they look up from their Pokemon daze & see books, it would be a chance to get one caught :o) As long as it's safe & it works, it's good! My local library is a PokeStop, so they are making a connection between books & Pokemon - trying to lure an age group into the library that doesn't usually go there.