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.
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.