You should be able to download any Project Gutenberg text (as a .txt file), save it to your mods/gutenberg/books/ directory, and it will be available in the game. [You must be sure to give it a file name containing only letters, numbers, underscores, and periods, or the mod will not recognize it.] There is no special formatting, and images are not supported.
Note: The mod will create a cache file for each page of each book every time you start the game. This takes so little time on my ten-year-old laptop that I don't notice, but it could be an issue for someone. If you don't like your world directory cluttered, you may create a subdirectory in it called 'book_cache'. Cache files will be placed there instead.
The source is available on github.
Code: LGPL2, textures: n/a
Mod dependencies: default
Download: https://github.com/duane-r/gutenberg/archive/master.zip
Recipes:
Combine the following to produce a random gutenberg book:
Paper, [empty], [empty]
[empty], Paper, [empty]
[empty], [empty], Paper