most books have a safeguard that stops the player from reading if the book doesn't grant any benefit. for example, reading the angler's survival guide with no water nearby will stop the player from losing sanity and book durability.
if the lunar grimoire is read during the night of a full moon the book has no safeguard and will cast regardless, not being consistent with the other books
1. read lunar grimoire on the night of a full moon
2. watch as it casts and a book use and sanity is wasted