I can think of two primary resources for information and pictures of the Remington 7188. They are the following:
The World's Fighting Shotguns by Chief Warrant Officer (ret.) Thomas Swearengan. It was published by Ironside Intrntl Publishers around 1979. Best amount of information in regards to its development and good b&w profile pictures.
Special Warfare, Special Weapons (vol.1) by Kevin Dockery. Published by Emporer's Press in 1996 or 1997. Has the only in-country photo of a 7188 issued (on a PBR), and it is pretty much a silhouette of the gun (poor light). I think Dockery's book offers more specs on it in a more organized fashion (as well as a photo of a 7188 being range tested on FA by him), as well as more information as to the troubles surrounding it. But Swearengen has the best photos of the gun itself (including a comparison photo of the 7188 and the 1100's trigger assemblies to display the differences).
Dockery also has a multi-use Role Playing Game weapons sourcebook outlining a variety of weapons and their specs titled Edge of the Sword that I think was published in '91 or '92 by R. Talsorian Games (Dockery created a role playing game called "The Morrow Project).