Since moons orbit planets it's sort of like they have a fly on the wall perspective. Moons know the secrets their planets keep. His library is full of books each representing a different moon in the universe. So planets with several moons have a 'series' of books.

The books are sorta metaphysical because they don't have words; they just allow you to see inside of that moon's mind. Each secret is a memory from the moon. Since individual celestial bodies can have their own god(s) it's important to keep records of stuff. New planets could become new conquests for established gods. Books are constantly floating around. Changing location and such. Well when they stop being a planet's moon the book will end. If they become a moon to a different Celestial body then a new book is created.

Some gods that come to look at books call him librarian. He's very undervalued. Other gods appreciate his work and bring him things from their world's or a world they preside over. Since Tsuki can only read about other cultures he really loves getting to actually physically see things. He can get an idea of things from the books but having the item is different from seeing it. It's a library mainly for the gods and for record keeping. Knowing a planet lived; even if it gets sucked into a black hole. Most moons don't have any secrets to tell. So it's mainly a timestamp of when it started orbiting to when it stopped. There’s a dark section of the library dedicated to moons that crashed into their planets.