The Threshold Universe is an ongoing book series written by Peter Clines and begins with the novel 14 published in 2012. The other books in the series included The Fold (2015), Dead Moon (2018), and Terminus (2020). Although 14 and The Fold were published both as paperbacks and audio books, Dead Moon began as an Audible exclusive and after its initial run on Audible, it was published as an e-book while Terminus has been published exclusively as an Audible Original. All of the audio books for The Threshold Universe have been published through Audible and all are read by Ray Porter.