Hello, release 2.0.1 is missing at http://dogforce-games.com/maven/gigaherz/guidebook/, therefore this version can't be used with gradle

Oops will upload asap.

offtopic - how do I add a book to a mod? The way you suggest in the manual is unusable since I can't import your package.

    @Optional.Method(modid="gbook")
    @SubscribeEvent
    public static void registerBook(final BookRegistryEvent event) {
        event.register(new ResourceLocation(location));
    }

Why can't you import my package? Even if you can't add it as a maven dependency, you can still load it as a fileTree dependency...

I see, I believe that's because guidebook depends on gigaherz.commons, which I use in my own projects as a dependency, and I include as an embedded jar within the guidebook jar. You may need to add this dependency also:
deobfCompile "gigaherz.commons:gigaherz.commons-1.12.1:0.6.4"

Good to go, hopefully, you will find some time to continue at this free time job.
Have a nice day :)

I finally got it, it's imported as a maven dependency, but this occurs when running gradlew runClient with a mod, but not as the standalone mod, (also tested with locally compiled version of Guidebook)
crash-2018-05-18_09.03.01-client.txt
Edit: runClient from your repository with mod added as local dependency works (except it does not give you a book after world generation) but in opposite direction doesn't