There are 4 books in the The Science of Discworld series. The newest book is Judgment Day which came out in 2015.

1
The Science of Discworld

By Terry Pratchett , Ian Stewart , Jack Cohen

Book cover of The Science of Discworld
When a wizardly experiment goes adrift, the wizards of Unseen University find themselves with a pocket universe on their hands: Roundworld, where neither magic nor common sense seems to stand a chance against logic. The Universe, of course, is our own. And Roundworld is Earth. As the wizards watch their…
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2
The Globe

By Jack Cohen , Terry Pratchett , Ian Stewart

Book cover of The Globe
The acclaimed Science of Discworld centred around an original Pratchett story about the Wizards of Discworld. In it they accidentally witnessed the creation and evolution of our universe, a plot which was interleaved with a Cohen & Stewart non-fiction narrative about Big Science. In The Science of Discworld II our…

3
Darwin's Watch

By Terry Pratchett , Ian Stewart , Jack Cohen

Book cover of Darwin's Watch
Roundworld is in trouble again, and this time it looks fatal. Having created it in the first place, the wizards of Unseen University feel vaguely responsible for its safety. They know the creatures who lived there escaped the impending Big Freeze by inventing the space elevator - they even intervened…

4
Judgment Day

By Terry Pratchett , Ian Stewart , Jack Cohen

Book cover of Judgment Day
The wizards of Unseen University are again called upon to defend their creation, Roundworld, this time in a courtroom—where its very existence hangs in the balance.
 
The Omnians fervently believe that the world is round, not flat, and view the discovery of Roundworld as a vindication of their faith. To…