There’s an excellent matrix at http://epubzone.org/solutions that allows you to search tools for the solution that best meets your requirements, eg ePub2, ePub3, fixed layout, reflowable, self-publishing.

There are some fantastic expensive/production tools for creating eBooks. If money was no object I’d be using:
Oxygen XML
Namo

When it comes to free or low-cost solutions, I’d go for:
Calibre
Sigil
Smashwords

Some other excellent resources are:

https://ebookflightdeck.com/handbook
http://www.cnet.com/how-to/how-to-self-publish-an-ebook/

The process I ended up using was simply to create a document using Word, with manual page breaks.
I then imported it into Calibre, added a cover, and published it as ePub2.
The main reasons for this approach were:
speed – it had the shortest learning curve
popularity – Calibre comes up in the top two open source methods on every article
cost – it’s free
layout – I didn’t want to restrict it to fixed format, even though that it the most popular option for children’s eBooks.