In your quest for eBooks, you may find that there are many eBook formats. These are the most best eBook formats (and most popular) you will find in your quest to e Reading.
EPUB 
The ePub format is an open ebook format designed by the Open eBook Forum and developed by the International Digital Publishing Forum.
Based on XHTML and XML, it was made with the intention to be a both source file format and end user format. Examples of software that can open this format are EPUBReader Firefox add on, Adobe Digital Editions, and QuickReader.
Meanwhile, devices that can open this file include the iPhone, iPod Touch, iPad, Sony Readers, Kobo Reader, and the Nook from Barnes & Noble.
AZW 
This is an Amazon ebook format used exclusively on the Amazon Kindle. It is basically a MOBI format that uses a high compression option. Kindle apps have been released for devices other than the Kindle reader.
AZW format files can also be opened on smartphones (iPhone, Android phones, and BlackBerry), computers (Mac and PCs), and tablets (iPad, Android, and Windows 8/10 tablets) using the respective Kindle Apps.
PDF 
The Portable Document Format was created by Adobe for its Acrobat products. This is a probably the most popular ebook format since software support for this format exists for almost any device – PCs, Mac, Linux, iPhone, iPad, Android phones and Tablets etc.
There are many PDF viewers in the market, including Adobe Reader, Foxit Reader, Nitro PDF reader and otherm Most of which are free.
One downside of this format is that content in PDF is usually scaled for A4 or letter size, which becomes very difficult to read when adapted to small screens of smartphones or other eBook readers, such as Amazon Kindle.
MOBI 
This ebook format, used by the MobiPocket Reader, MOBI was originally made as a PalmDOC format’s extension. It can be opened using MobiPocket’s reading software, which can be installed on a lot of PDAs and smartphones.
There are limitations of the MOBI format. For example, sizes of images cannot be scaled with the font size; tables display differently on different readers, especially if the table covers more than one screen; and there is an absence of support for nested tables.
Third party readers such as Stanza, FBReader, Kindle for PC and Mac, and STDU Viewer can open MOBI files.
One thing that an e-book user should know is that formats can be unprotected or protected by a digital rights management (DRM) wrapper. The DRM, which is used as an anti-piracy tool to protect authors, can prevent you from opening an e-book file even if the format is compatible with your reader. Amazon Kindle books are all DRM-protected. Also, many PDF and ePUB eBook from publishers might be DRM-protected as well. Apple iBooks is one of them.