It's a book of everything that has been forgotten. It appears in the Pilot with the same name and appears in the final chapter of Over the Garden Wall.

Tome of Unknown

It was planned to be an important Object in the Pilot, as the Narrator made it obvious the object exists. It was eventually scrapped in mini-series, possibly because of time constraints.

Wirt, Greg and Beatrice hope to find this book in the Tome of the Unknown. - Pilot Narrator

Over The Garden Wall

Put however the book did only appears in final episode of Show with Lorna reading it.