There are a few things I'd like to mention, both if you're looking to learn more about .epub and if you no longer want to deal with it.
Although they may be a little longer, you are always welcome to view the official documents on .epub over at idpf.org/epub. It goes much more in depth and hopefully you won't feel as lost after seeing this preview!
Additionally idpf has a github page github.com/idpf/epub3-samples, where they host many other example ebooks! Many of these are used to show of different techniques one can use, so they may not directly help build your ebook off this site, but will help with creating a whole new one!
And lastly they support a web page http://validator.idpf.org/ which may be used to check your ebook for possible errors or issues. There's also a similar system here where you may download the checker onto your computer and vefify books >10mb.
Calibre is, although not the prettiest, arguably the most fully-featured free resource to use when converting and using and kind of ebook. I highly recommend you check them out if you're planning on converting or using your ebooks. You may visit their site here https://calibre-ebook.com/.
You may also check out another one of my personal pet projects, https://github.com/thansen0/EBookCompiler. Although at the time of writing this it's a work in progress, you will be able to run it with text or html files and it will compile them into an ebook for you. The direction I'm planning to take it is rewrite it in ruby and turn it into a gem for websites to use, however as it stands now the java app should work fine for creating ebooks or personal projects for now.