Vitaly Pedchenko, who runs the forum and blogs at Rem870.com, has written a 270-page ebook on the Remington 870 platform. The book is packed full of photos (one of the benefits of an ebook-only publication), tutorials and interviews with famous shotgunners. It only took a few minutes of reading it before deciding I was happy to recommend it to y’all. The PDF costs $10, which I think is a bargain.
The Kindle version is cheaper at $5, but I recommend buying the PDF (which is the version Vitaly sent me) as I have had bad experiences reading photo-heavy books on my Kindle.
More information including a Table of Contents can be seen at Rem870.com.