Since Rain had some time to kill, he decided to test a bit what the shield said about what he could do with the domain… that was one way to experiment with the limitations of the system, as well.
Rain realized that creating books within his mental landscape to contain the knowledge he possessed required minimal effort. Using a moderate amount of mana enabled him to materialize a book, and with a slightly additional expenditure of mana, he could replicate or transcribe the specific knowledge he desired into these books. The convenience and ease of this process within his mental space were obvious to Rain.