Hi Albus
Whilst using your eBook. I have made a few notes for you.
Navigation:
Your Navigation throughout your website is easy to identify and navigate. However, I found that the placement of your controls moved around from page to page. I would consider making these all uniform and in the same position to make the experience as easy as possible for the user. I also believe that the size of your buttons was different from page to page.
Color:
the color pallet throughout your pages is different. I would consider having one highlight color when you mouse moves over a button. This will allow for the user to easily navigate your site. been able to do this will assist in reducing the cognitive load required to navigate your book. this is important to increase the user experience. the user doesn't want to have to work for the content they want it handed to them on a silver platter.
keep it these few adjustments will go a long way in creating a better UX.