Psyren: Another Call

A light novel written by SOW was released in 2010 along with volume 13. The covers from volume 13 and the novel combined to create a full picture. The first novel, called Psyren: Another Call, includes various short stories about the characters, including one about Ageha going on a date with Sakurako and Abyss; Kagetora's first meeting with Matsuri; a chapter about Amagi Miroku; and a chapter about Frederica in the future.

A second light novel was released with volume 16 this one focusing on different epilogue stories that detailed the futures and lives of the characters and as thus giving a proper close to the story.

Source: Psyren Wiki