Looped Portal

Looped portal is a pretty useless thing, but we should palce it here. Take two same books, go to book's dimension. Dimension's portal will be standard nether portal. But if you will drop your second book you will get a portal which teleports you to himself. Maybe it can be used somewhere.

Usage found
Looped portals are pretty broken, so when you trying to drop another book, only half of the portal will update. But both sides are active.