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).
Basic Knowledge
Before you can go details, you have to learn the basics about operating system architecture. Yes, really.
Operating Systems Basics
Books. Etc.
GNU/Hurd System Architecture
Capabilities
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.
Our use of capabilities. The Wikipedia, capability-based security article. Alternatively/additionally, [shapiro_capintro_1999].
In the GNU/Hurd system, a capability is represented by a Mach port.
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.
FAQ
Specific Components
Documentation for...
Presentations
2004
2004-07-02
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.