D is for...Design Contest
Readers are more engaged when they have a stake in the content. Whenever possible let your readers offer input or make decisions in various aspects of your book.
If your self-publishing, consider posting several different covers and let your readers vote on which one to use. Offer a contest to have reader names included in your next book. You can open this up to anything from title to setting cities and everything in between.
For example, what Hunger Games fan wouldn't have bent over backward for the chance to design an outfit for Effie in the second book. And Harry Potter fans would have handed over their first born child to invent a spell to be used in a book.
Not comfortable giving readers control of your content? Let them get creative in your marketing. Offer readers contests to design bookmarks or a tour banner. There are dozens of options here, maybe even hundreds. You're only limited by your comfort level and imagination.