Curtis Bonk is a well-known author of books related to distance education and online technology, as well as open education. He has recently co-authored a book called Adding Some TEC-VARIETY: 100+ Activities for Motivating and Retaining Students Online.
Bonk has written a similar book in the past with Ke Zhang (Empowering Online Learning: 100+ Activities for Reading, Reflecting , Displaying and Doing, 2008), modelled on a particular approach to instructional design.
In this book, the focus is specifically on motivation and retention of online learners, and it is based on ten established principles of motivation.
Best of all, you can download a pdf of the book free of charge. Just visit the book website for Adding Some TEC-VARIETY.
We’ll be planning a few activities in Spring 2015 to discuss some of these ideas.