Piggyback Publishing, Piggyback Publishing Review
Piggyback books thrive in a wide variety of niches, categories, and demographics.
https://www.youtube.com/watch?v=Gh6_P5Kp_dY
http://www.scoop.it/t/bring-the-fresh/p/4054667888/2015/11/05/piggyback-publishing
http://www.bitlanders.com/blogs/piggyback-publishing/3680127