In singleplayer, opening a book which is displayed on a lectern does not pause the game, but opening the book in the inventory pauses the game (
The behavior should possibly be consistent. Ideally both actions should not pause the game because at least for the lectern it is required to use its redstone functionality.
- Set up something which indicates that the game is running, e.g. a redstone clock
- Open a book in your inventory
→ The game is paused
- Put the book on a lectern and open it there
→ The game is not paused