DC in a nutshell

If one wants to find out about Direct Combination quickly, the Mobile HCI 2002 paper DC Holland 2004 Springer.pdf might be a good place to start, followed by BCS HCI 2004 HCI2004HollandDC.pdf - though if in a real hurry, just read the latter.
To consolidate this, the 2010 Presentation offers a useful overview.
Then if you want to find out more about particular aspects, there are some very short papers that reflect on particular aspects of Direct Combination, such as: "A Simple Taxonomy of Search Reduction", Simple Taxonomy DC Holland.pdf "When is Direct Combination Useful" When is Direct Combination useful.pdf, "Extended Direct Combination", and "View points in Direct Combination" Diverse viewpoints in Direct Combination.pdf.
The Spontaneous Interaction paper Spontaneous_TR.pdf is fun, too.

The original CHI paper CHI99DC.pdf is fun, and a good read, and is recommended, but it is important to realise that the framework and programming mechanisms are completely different in the version from 2002 onwards. The original implementation based on class based decomposition and inheritance mechanisms proved too brittle to allow DC to be really flexible, but the change to a role-based system in 2001/2002 transformed what was possible. N-fold interaction and viewpoints became simple, and user-driven incrementally-developed systems became straightforward.

Improved mechanisms were made possible by ideas from the late Henrik Gedenryd's Universal Composition - a radical new way of organising computational objects. Henrik was adamant that this could make viewpoints easier. Simon Holland worked out a mechanism to re-conceptualise and implement n-fold Direction Combination in a scaleable fashion. Simon noticed that Universal Composition was conceptually isomorphic to Trigve Reyskaug's work on OORAM. To find out more about these mechanisms look at "Meeting the software engineering challenges of interacting with dynamic and ad-hoc computing environments" Dynamic and ad-hoc.pdf, and OOPSLA OOPSLA 2004 RC Holland Final.pdf.