Loading up dark utilities and bookshelf together or separately with a fresh install of forge 1.15.2 will crash the game. Sorry, I have no idea if its because of this mod, bookshelf, both or something else. O.o [VVV Crash Report is below. VVV]
crash-2020-03-26_14.14.06-client.txt

Thanks for reporting this issue, you need to update your version of Minecraft Forge as Bookshelf now uses new features in Forge.