Searching and Accessing Data in Riak by Andy Gross and Shanley Kane.
From the description:
An overview of methods for searching and aggregating data in Riak, covering Riak Search, secondary indexes and MapReduce. Reviews use cases and features for each method, when to use which, and the limitations and advantages of each approach. In addition, it covers query examples and the high-level architecture of each method.
If you are already familiar with search/access to data in Riak, you won’t find anything new here.
It would be useful to have some topic map specific examples written using Riak.
Sing out if you decide to pursue that train of thought.