Help with equipment nbt

So I'm trying to detect a written book both in the offhand and mainhand.
When I serch for it in equipment.mainhand it doesn't show up but if i search in equipment.offhand it does.
How does the equipment work, does it only accept certain items in certain slots?
**What would be the best way to detect any item in either offhand or mainhand? **
would it be a combination of equipment.offhand and SelectedItem?