In the present essay, open books refers to a journal (in the bookkeeping sense) that is SQL (or similarly) queryable by anyone. The creation of a cooperative business or nonprofit organization implementing such a feature would be a fine long-term project for a PubWan movement, if a sufficiently large one ever materializes.
The article Transparent Accounting, by Robert L. Read and Mike McCune, proposes a global namespace of accounts. While not as aggressively "open books" as defined above, implementing it would be a very relevant and useful PubwanActivity, for example as a shared database of PubwanParticipant NormSpec's.