C++'s canonical reference is a very thick book. I figure there is a conflict of interest between the published version and presenting an equivalent online.

It's worth noting there is no good C documentation in one place, either.

Python dumped the book quickly and went online, but that was probably more for the publisher not being interested in releasing new versions every time something changed. It's also a little younger, but still appeared before the widespread use of broadband internet.

I think Perl was probably the first one to make documentation a real first class citizen, with perhaps Emacs shining a light in that direction.