Introductory Material
The Hurd and Linux, a comment by Richard Stallman.
Towards a New Strategy of OS Design, an architectural overview by Thomas Bushnell, BSG.
The Hurd, a presentation by Marcus Brinkmann.
A document about translators by Marcus Brinkmann.
A Critique of the GNU Hurd Multi-server Operating System, an analysis of the GNU Hurd on GNU Mach system, written by Neal Walfield and Marcus Brinkmann.
External
Examining the Legendary HURD Kernel, an article by David Chisnall.
Also covers a bit of GNU's and the Hurd's history, fundamental techniques applied, comparisions to other systems.
Development
The Hurd Hacking Guide, an introduction to GNU Hurd and Mach programming by Wolfgang Jährling.
Manually Bootstrapping a Translator, a text by Neal Walfield about how to manually connect the translator to the filesystem.
The Authentication Server, the transcript of a talk about the details of the authentication mechanisms in the Hurd by Wolfgang Jährling.
The Mach Paging Interface as Used by the Hurd, a text by Neal Walfield.
In the Position paper Improving Usability via Access Decomposition and Policy Neal Walfield and Marcus Brinkmann give an overview about how a future, subsequent system may be architected.