Option for author to "theme" thier book with 5 or so different layout themes so one would be fictional, another might be non-fiction (using some sans serif font), etc. Of course the reader can override if desired.
The book title at the top gets lost in the content.
The top menu symbols show up on the printout - might want to have the print version have a light BookVoid header showing... with the book title.
Maybe an option to border embedded graphics, that wake turbulence pic doesn't seem to rest well in the page... (can you caption it too? or have text wrapping options?)
When you go to settings the book content graphics disappear.
The navigation symbols are interesting but I would think trying to be a bit more conventional symbols (+/- magnifying glass, graphic of gate-fold, etc) would be worth a shot.
BookVoid? Sounds like limbo from books... Maybe instead of a word that reminds me of emptiness or clearing out, look for something growing or spreading... maybe look for Book[Word] combos relating to say garden, spring, sharing, inspiration,creativity?