An Adobe InDesign template perfectly suited for e-books, reports, or any kind of longer documents.
It comes with a lot of built-in assets: character styles, paragraph styles and object styles for titles, subtitles, table of contents, index, footnotes, (anchored) sidenotes, quotes, code blocks, note boxes, image captions, basic tables, running headers and master pages. A great document to learn InDesign from too.
The design itself is simple and clean, with a focus on solid typography. This allows you to easily change and tweak it to your taste. Also everything is aligned to a (baseline) grid.