HyTime overview and links A Tutorial Introduction to SGML Architectures . HyTime Application Development Guide . Hard going, but detailed and pretty clear. Quick Guide to HyTime Basics . A useful summary, mildly specific to HyTime 1st edition.
Groves are a central concept, abstract rather than inherently difficult.
There are several exellent discussions of groves in the appropriate SGML special topics section. Here is a selection. This same abstract data view could be used by any other processor, standard or proprietary. Derek Denny-Brown . The Purpose of Groves , Steve Newcomb.