Vim: The Basics by Andrew Stanton is an introduction to Vim, costs $8, and is available for Kindle, ePub, etc.
This eBook guides you through the basics of Vim, helping you to become familiar and competent with one of the most popular text editors in the world.
What I really liked about this book, though, is the author asked for feedback on a reddit thread: VIM eBook feedback request (self.vim).
I believe that the main sentiment is that there is no real market for this book. That existing free multimedia content from more experienced VIM users provides a better more enriching experience.
There are some excellent comments on that thread, particularly from seasoned veterans who are used to helping newcomers:
from years of reading and answering questions in #vim (irc.freenode.net) I learned two things. The documentation won't help if the user doesn't know what she's really looking for (because she doesn't know about Vim's vast amount of features). Moreover, often people find or use pretty hacky solutions just because they don't know better.
He also recommends Practical Vim by Drew Neil, which is a fresh take on the Vim book which I have read and enjoyed.
If you actually wanted to write an interesting book that might sell, the great unwritten vim FAQ book would be a handy book to have nearby. Go through the vim-users mailing list, hang out in #vim in freenode, put together a giant list of the most common issues and write up detailed, clear answers to them. I have run the #vim channel on Freenode for just a hint over 12 years now, and the questions are exceptional repetitive.
Trawling mailing lists and chat rooms for themes of problems is one of the techniques I've used for writing content about programming. I find it's more interesting to see people's problems with real details rather than abstract solutions in FAQs and wikis.