Seconded. I heartily recommend anyone with even a slight interest in Riak to check out the Riak Fast Track:

http://wiki.basho.com/The-Riak-Fast-Track.html

It's a quick, interesting, and to the point into to Riak, including theory, installation, and usage. The Basho guys have seriously great documentation, I've enjoyed browsing through the Riak wiki despite not really having any live Riak deployments.