The Planetmath version adds a lot of hyperlinks (automatically). Although many of their links are wrong (particularly because they go to elsewhere in planetmath rather than to appropriate places in the book itself), it occurs to me that there is similar value we could add ourselves: the online-PDF versions of the book could be hyperlinked with hyperref. That is, rather than making the reader go look up "quasi-inverse" in the index to find its definition, the 21st century way to do it would be to make "quasi-inverse" a hyperlink. Then anyone in the future converting the book to HTML would also have information bult-in to the source about what links should be added.
This would be a lot of work, but I think it's worth thinking about -- maybe some reader(s) of the book will want to take it on as a project.
It's not necessarily that much work. We should be able to reuse the indexing tools.