If you are considering building your own Open Educational Resource; there are tools to help you create an ebook
that can be read on multiple platforms (tablet, pc, phone, mac, etc).
Several guides are available to help open textbook authors:
Pressbooks: This web-based software lets you create ebooks in a variety of formats: PDF, Kobi (for Kindle), Epub, and others. The basic version is free (after you register). Subscribers get premium features. This software is a product of WordPress.
These is an example of a textbook that was built using Pressbook by faculty at Cleveland State University for a Freshman English seminar. The Faculty found they needed to teach students skills on composition, thus the OER resource was created.