If you have a Remington 870 shotgun, you might want to check out this e-book from Vitaly Pedchenko. Called the Remington 870 Shotgun Guide, this is a solid book that covers pretty much anything that the average 870 owner would ever want to know. Pedchenko runs the Rem870 site and forum.
The book is 270 pages long with plenty of full color photographs and illustrations. Pedchenko covers everything from disassembly of the shotgun to clearing malfunctions to reviews of the many popular upgrades for the 870 that are out there.