Whether you're creating ebooks to sell, or converting your own files to read on your ebook reader, you don't need to spend a fortune to get a great result. Here are a selection of free tools you can use to create professional quality ebook files.

Calibre

http://calibre-ebook.com
WIN | OSX | LINUX

Calibre is quite possibly the most useful ebook tool available today. Designed to be an ebook library manager, this tool does a better job of managing and sorting my ebook collection than anything else I've come across.

In addition to storing your ebooks, it can convert to and from multiple format, turning text files, Word files, and even old Microsoft Reader files into formats that can work with whichever reader you have handy. It does a great job converting from EPUB to a Kindle-compatible .MOBI format, and includes a built-in EPUB editor to fine tune your files.

One of my favorite features is the metadata editor, which makes sure your ebook files contain all the information needed to make them indexable by major online ebook sellers.

Kindle Plugin For Adobe InDesign

http://amzn.to/2Dg1cb6
Free InDesign Plugin for WIN | OSX

While it is certainly easy to convert your EPUB using a third-party tool like Calibre, if you've created your print layout with Adobe InDesign you can use Amazon's plugin to export your ebook directly into their new Kindle 8 format. With this plugin you can add your metadata, format your table of contents as well as adjust your image quality.

Sigil

https://github.com/Sigil-Ebook/Sigil
WIN | OSX | Linux

Sigil is a free, open source editor for EPUB e-books which can be used to create new EPUB files as well as edit the existing EPUB files.

While Sigil does require some knowledge of HTML and CSS to use, it can be an invaluable tool when troubleshooting and editing your EPUB files for formatting and validation errors.

KindleGen

http://amzn.to/2DgsT3f
WIN | OSX | LINUX

If you have an EPUB generated from any other software program, but want to submit your title as a Kindle ebook, you can use Amazon's command-line tool KindlGen to turn your EPUB into a Kindle Format 8 (KF8) or MOBI formats for submission to Amazon's Kindle ebook store.

EPUB Validator

hhttp://validator.idpf.org/
WIN | OSX | LINUX

No matter what software you've used to generate your EPUB file, you will want to validate it before sending it off to your distributor or bookstores. The International Digital Publishing Forum offers this web-based validator to check your files (below 10 MB in size) for errors in OCF container structure, OPF and OPS mark-up, and internal reference consistency.