This could be due to several possible reasons:
1. Incorrect installation of mods: If you have installed mods that should affect the experience for reading books, they may not have been correctly configured or interact with other mods, which leads to undesirable results.
2. Lack of appropriate modification: If you are using a vanilla (unmodified) version of the game or mods that do not provide experience for reading books, in this case the experience multiplier may be disabled or not implemented in the game system.