It was some books like this that got me started learning the editor and how these ebooks really work. And how simple they
should be. It's a mystery how people even manage to build some ebooks, I can't imagine the tools that create such a mess, or why.
One like this, I'd take the dynamite approach and edit the html and css down to one or two very simple styles, remove all ids and <span>s, and change all those <div>s to <p>s. Great learning exercise if you want it. The other dynamite way is to convert the thing to txt, clean it up using paragraph styles in Writer or Word and convert from there. It's just a plain simple novel, so not much styling is needed.