Not to cast aspersions on either Lamport or the content of the work, but this "hyperbook" would be much more accessible to contributions as a Github repository, and much more accessible to readers as an ePub (or plain hypertext!) The approach here feels more like what you'd come up with to open-source a book's production in the 1980s.
A single big HTML file with MathJax and judicious use of semantic tags seems to me like the way to go. Heck, maybe even all Markdown with a good Pandoc build script included.
