I think IBM came up with SGML way back when, on which many of our modern markup languages are based, or were based originally. I used IBM's BookMaster (General Markup Language) in the 1980s to write and format technical software manuals for IBM. That is probably the grand-daddy of book formatting languages and of coursed based on SGML.

HTML was originally SGML based as well, and still shows its roots. HTML is of course the web based document format, but it wasn't designed for printed material or eBooks, much less multimedia.

I know at least for a while Palm OS used PML (Palm Markup Language) for eBooks. It was a very simple markup language without many bells and whistles, but you don't need much for rendering novels I guess.

Mobi format came along and was used by Fictionwise books. But it was still rather limited.

The ePub standard is supposed to be "THE" standard. It is basically HTML modified to work for eBooks. But since Amazon sells 2/3 of all eBooks in the USA market, whatever they use trumps the so-called industry standard, like it or not. KF8 is based on the ePub standard, BTW. From what I understand it is a modified ePub.