Transworld Publishers have announced that two previously unfinished books by Sir Terry Pratchett will be released in collaboration with author Stephen Baxter.

In 1986, Pratchett started work on a series of stories entitled The Long Earth, which envisioned a chain of parallel worlds existing side-by-side. Working with Pratchett, Stephen Baxter, author of Flood, Ark and the Time’s Tapestry and Destiny’s Children series, is bringing this long-gestating concept to light in the form of two as-yet-untitled books.

Transworld released the following description about The Long Earth: “Our Earth is but one of a chain of parallel worlds, each differing from its neighbours by a little (or a lot) in an infinite landscape of infinite possibilities. And you can just step from one world to the next…”

The first Long Earth novel is due for release in spring 2012, from Doubleday.