Perhaps the prototypical (stereotypical?) example of a "compound document" system is OpenDoc , which assembled documents as collections of "document objects."
This approach to documents generally involves the use of object technologies such as CORBA.
Rather than building an "application," you build up objects that know how to manage some sort of document component, such as a spreadsheet, picture, chart, table, or whatever. The user then embeds documents as needed to build up the document that they are working on. OpenDoc's storage system was called Bento; it is not clear that it was ever fully released.
OpenDoc builds up documents using "object components."
Note that the OpenDOC initiative has essentially failed from a commercial perspective, and IBM has decided to make IBM OpenDOC Sources "freely available." See McCusker discussions on OpenDoc
The GNOME and KDE projects are seeking to ultimately build somewhat similar "compound document" infrastructures atop CORBA; we'll see where that takes us...
Open Parts --- a free object model for Unix (wants to be an alternative to DOM/Baboon/KOM)
Bonobo is a set of language and system independant CORBA interfaces for creating reusable components (controls) and creating compound documents.
Bonobo Documentation:
CORBA and the GNOME; intro to Bonobo
On the use of CORBA in the GNOME system's compound document framework, Bonobo.
Bolide (being worked on by one of the authors of Bento is a portable, cross-platform software development framework of the object oriented component software variety, for building compound documents and other types of content centric component software. The Bolide model aims to break applications into semi-independent parts that can be put together in different ways by users or developers to create various end applications. Bolide normalizes an application environment so such parts can live together in the same space without conflict. Bolide standardizes and arbitrates inter-part communication so common resources can be shared in accord with application framework patterns. Bolide is a redesign of component software from first principles, vaguely similar to systems in Microsoft's OLE, Taligent's CommonPoint, and Apple's OpenDoc. But Bolide aims for goals more modest in scope.
Apparently, as a "clone" of Apple's Publish and Subscribe system...