Note: The Usage Manual has been moved to the GNU Radio wiki, along with all other documentation that is not specific to a block/class/function/etc.
For viewing offline, snapshots of these wiki pages are periodically exported and stored in gnuradio/docs/usage-manual/
Here are links to the various Usage Manual pages (they are also linked on the home page of the wiki):
Getting Started
Metadata and Messages
Advanced Development Topics
Signal Processing and Digital Communications
VOLK