[2023] MOVED: How do I know if I already have read a book?

Originally posted by Dr. Rockso: The icon will look like an open book if you have already read it.

This. The small icon in the corner of the item is a closed book if you haven't read it, and an open book if you have.

Take note of those icons in the corner- They all signify something, For example, a gear icon is a modded item, an explosive icon is something that goes boom, a trio of shapes is a building block with multiple shapes accessible via the shape menu, etc.