Classic Daggerfall books are located in the arena2\books folder. They can be edited with a tool named DFBOOEDT, available as a component of DFTools package, which can be found at Uesp. These tools are created and mantained by groups aiming to localize Daggerfall.
Daggerfall Unity imports books from the original game local installation, but overrides can be provided as loose files or bundled in a mod to benefit of load order and simple un/install, without permanently affect core files.
- BOK*.TXT.byte inside a mod as a byte asset.
Import of additional custom books is currently provided as an experimental feature. See forum post for details.