Hypermedia is certainly one of the constraints in REST. Roy Fielding makes that very clear: http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven.
Try also to search for "hypermedia" inside his thesis: http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm
This blog post tries to explain the benefits of hypermedia: http://soabits.blogspot.no/2013/12/selling-benefits-of-hypermedia.html (disclaimer: I wrote it).