You are interested in getting familiar with the GNU/Hurd system architecture, or specific components of it? Here is a collection of texts to read.
A lot of stuff is still missing (open issue documentation).
Before you can go details, you have to learn the basics about operating system architecture. Yes, really.
shapiro capintro 1999: What is a Capability, Anyway?, Jonathan Shapiro, 1999. This is an easily readable introduction with good examples. In the author's own words, the text provides a layman's introduction to capabilities, describing what they are, what they do, and why they result in better security than today's computer systems.
Wikipedia, object-capability model. Albeit not directly tailored at the GNU/Hurd operating system architecture, this artice anyway does a good job at describing general principles of a capability-based system architecture.
You may want to start reading a quick overview of how the GNU/Hurd system starts, and thus how it is structured.
The GNU/Hurd system is composed of the Hurd servers running on top of the GNU Mach microkernel. Mach only handles scheduling tasks and threads, memory management, and message passing. Mig is used to abstract RPCs on top of Mach's message passing. Eventually, the Hurd servers and Unix processes discuss with each other using these RPCs.
Depending on your background, you may want to start reading the documentation from userlandish interfaces (Hurd) or from the micro kernel itself (Mach).
- FOSDEM: presentation by Samuel Thibault: Along the GNU Hurd RPC way -- A starting guide to contributing to the GNU Hurd (slides)
- Debconf 15: presentation (including video) by Samuel Thibault: Debian GNU/Hurd Status update (slides|video|transcript)
FOSDEM: presentation by Samuel Thibault: The GNU/Hurd architecture, nifty features, and latest news (slides)
- GNU Hackers Meeting in Paris: presentation by Samuel Thibault: GNU/Hurd, aka. Extensibility from the Ground (slides, video)
GNU Hackers Meeting in the Hague: video of the presentation by Neal Walfield: GNU/Hurd: It's About Freedom (Or: Why you should care)
Fosdem 2010: Why is Anyone Still Working on the GNU Hurd? (Sun., 10:30, Alt-OS Developer Room)
- Fosdem 2010: Porting KGI graphics drivers from Linux to GNU Hurd (Sun., 13:00, Alt-OS Developer Room).
Ognyan Kulev, presentation of the Hurd, at the seminar LIO and friends, http://debian.fmi.uni-sofia.bg/~ogi/hurd/liofest-20040702-hurd.ppt, in Bulgarian.