The current firmware has an option for a full-screen reading mode which has no header (title) or footer (progress indicator). So the amount of unusable space depends on which format of book you choose (kepub or epub) and which reading mode you choose (full-screen or normal):
- EPUB in full-screen reading mode: no unusable space at all, every single pixel of the screen is avaliable for book content.
- KEPUB in full-screen reading mode: a small (2%) minimum margin on all four sides.
- EPUB in normal reading mode: a tiny (1%) minimum top margin and a very large (8%) footer.
- KEPUB in normal reading mode: a very large (8%) header and very large (8%) footer, and small (2%) minimum left/right margins.
There are patches which can remove the minimum margins in kepubs, and reduce or remove the normal-mode footer in epubs and kepubs, but there is no way to remove the kepub normal-mode header.