I am trying to buy a copy of Well-Tempered Clavier. What is the difference between the two formats: paperback vs sheet music?
The "paperback" and "sheet music" are identical; the sellers just labeled them differently.
ABRSM only offers one edition of the WTC I as see in their online store. However, note that there are two editions of WTC II, with one being double the cost of the other. Given the price difference in the Amazon listings, it's possible there is also a no-longer-available WTC I edition similarly priced.
I would suspect that the sheet music edition will be loose leaf so that it can be put on the stand of the keyboard instrument, whereas the paperback will be bound so it has a spine (like a paperback book).
They’ll be for different usages: the sheet music for playing from, the paperback for studying.