In today's world there are eBooks have two major formats: EPUB and MOBI. EPUB is an open source format compatible with most eReading devices and software (Apple and Android Smartphones, PC, Mac, etc.) There are a wide variety of apps that are usually free to reading the EPUB format. Adobe Digital Editions works well for PC or Mac, iBooks is great for iPhones and iPads, and Moon+ Reader and Mantano Reader are decent Android apps. EPUB is also the format distributed by all vendors (Nook, Smashwords, Kobo, etc.) with the exception of Amazon, which leads us to… The MOBI format is owned by Amazon and is used for Amazon-type devices such as the Kindle eReader, Kindle Fire tablet, and apps. Even though it is a different format, it is very similar to the EPUB format. The best way to make a MOBI eBook is to start with an open source EPUB eBook and convert it using Amazon's proprietary program KindleGen.
eBooks are not Print Books
A common misconception among publishers and authors is that an eBook format should mimic its print cousin as closely as possible. This is often a recipe for disaster. The content of a print book is designed to be read in a fixed layout (e.g. a 5"x8" paperback). However, eBooks must be readable on a wide variety of devices: from a large PC monitor to a small smartphone. Therefore, the content of an eBook needs to be "reflowable" so that it can adjust to the size of the viewing screen that the reader is using. Things like complex tables, images placed next to text, and background images may cause problems on eBooks and create a poor reading experience if not handled properly.