Hey man, did some testing (going through and adding every single mod one by one until your mod stopped working). Just wanted to let you know that the problem is Charm. I made a modpack with just your mod and Charm, and it doesn't let you put a book in the bottom slot. Delete Charm, and your mod works.

Is it possible to have Charm remove your code as he gives credit to your mod for it? I have tried to disable it in charm, but it did not help....

Is it possible to have Charm remove your code as he gives credit to your mod for it? I have tried to disable it in charm, but it did not help....

I don't think I can do much about it. Charm disables its module automatically if this mod is installed, but the mixins probably can't be disabled.

I think Charm has a very similar feature, so it makes sense that it conflicts. I don't know if I can fix that on my side, though.