Think of it as a white paper on steroids (the sexy white paper): a report, generally 12 - 40 or more pages in length, that presents complex information in a visually attractive, reader-friendly format. The content is both informative and entertaining; the tone, collegial; the format, "chunky" rather than linear, to facilitate skimming and scanning.
Develop your distribution strategy early: how will you get your ebook into reader hands? Think visually: make liberal use of bullets, callouts, sidebars, graphs, etc. Conclude with a solid call to action: what should readers do next?