What happened to the book of Earth?

There doesn't appear to be any in-canon account of what happened to the original Book of D'ni/Earth. However, assuming it follows the usual rules for D'ni books, it must still be intact, or the linking books to D'ni/Earth would cease to function.

That said, I believe it's logically sound to assume it would be possible to bring the original descriptive book to Earth.

• Remember, books only create the link between worlds - the worlds are not dependent on the books describing them.
• Linking books (at least the standard D'ni sort) are dependent on the existence of the related descriptive book. However, the location of the descriptive book does not matter. Linking books to Age A, residing in Age B, may be dependent on a descriptive book in Age C.

Therefore it should be possible to write the Book of D'ni/Earth in Garternay, then write some linking books that also go to Earth, then take the descriptive book to Earth through one of the linking books. The linking books still depend on the descriptive book, but it now resides in the age it describes.

The one major "gotcha" here is that you can't use books to link intra-dimensionally, so you couldn't actually use a book while it resides in the age it describes. The linking books in other ages should still work fine though.