Was working fine up to 2.0.0.425 of Bookshelf, now it tells me that the mod is missing.

Thanks for reporting. This should be fixed in the newest release. Please wait a few mins for Curse to approve the new upload. https://minecraft.curseforge.com/projects/dark-utilities/files/2469356

Tagging for tracking purposes
Darkhax-Minecraft/Bookshelf#88

the new version has the same error

An update was also needed for bookshelf. https://minecraft.curseforge.com/projects/bookshelf/files/2469466