Maybe we could start by adding a quick tour of the Gambit source tree and bootstrapping process to the manual?
According to https://www.gnu.org/software/texinfo/manual/texinfo/texinfo.html#HTML-CSS texinfo supports custom CSS stylesheets; that would hopefully let us make a good-looking manual for the web.
A book on Gambit internals would be very interesting but also sounds like a huge project. People who are experts on a topic sometimes write a blog over the space of a few years and then notice they have written enough material for a book.
We have an aggregator of all known Scheme blogs at https://planet.scheme.org/ (yes, the graphic design should be improved). I don't know how many people read these blogs (in aggregate or individually) but we seem to get about 10 posts a month.