If a mod is installed, which enables the possibility to craft bookshelfs for different wood variants, enable the possibility to do the same for added mushroom wood types.

Mods:

• Quark
• I like wood: https://www.curseforge.com/minecraft/mc-mods/i-like-wood
• Charm (has Bookshelf Chests, maybe also bookshelfs?) https://www.curseforge.com/minecraft/mc-mods/charm

Charm has no bookshelfs, only bookshelf chests.