This documentation is really LegOS documentation, since robos is just LegOS renamed (see why).
Warning: Many of the links below point to sourceforge pages that are maintained by others. I cannot ensure that the versions of the documents at sourceforge match the code here; if in doubt, compare the locally mirrored copy with the sourceforge one. LegOS documentation at SourceForge. This is the top LegOS documentation page at sourceforge, and an excellent starting point. Most of the documents below are direct links to documents that can be accessed through this page.
The legOS HOWTO. This is the most comprehensive and wide-ranging of the legOS docs, written by Luis Villa. The best place to start for a newbie. The document is also mirrored here: HTML, postscript, pdf.
Installation instructions for WindowsNT/Win9x platforms. This document covers the installation of the Cygwin tools and building of legOS on the Windows platform. Written by Paolo Masetti. Local mirrored copy.
Command Reference. Written by Dave Chen. Local mirrored copy.
legOS Kernel Documentation (postscript), written by Stig Neilsson. Local mirrored copy: postscript, pdf.
Auto-generated API docs. These are a set of API docs that are automatically generated from legOS source. Because legOS source code comments do not always follow the correct format, this API may be slightly incorrect or out of date. However, it is provided in case it may be found to be useful. Local mirrored copy.
Extreme Mindstorms (link to Amazon.com). If you want printed legOS documentation, this is the most up-to-date printed documentation available. It also details some pretty cool projects that can be done with legOS. The legOS section was written by Luis Villa, one of the co-maintainers of legOS.
Lego Mindstorm internals. This page contains a wealth of information on Lego Mindstorm internals.