Opus is a documentation mod. A book is just a folder of Markdown files in a resource pack, so there is no Java and no JSON to write. Folders become chapters, files become pages, and an index.md introduces its chapter.
Please use the GitHub issue tracker to report any bugs you find or suggest new features.
Obviously this mod would not be possible if not for the people at Mojang making an awesome game. Thanks also go to the developers of the Fabric and NeoForge mod loaders and the Architectury API.
Beyond those groups, huge credit goes to Vazkii and others who came before and created great mods like Patchouli, Modnomicon, and others.
This mod is released under the MIT license. The full text of the license can be found here.