SpagoBI code base is stable and well-commented, but it received little popular attention. The absence of books as well as the lack of case studies by third parties are among the possible causes for that.
Open source projects are planned, developed and maintained often using accessible Revision Control systems (e.g. Bazaar, CVS, Git, Mercurial or SVN), Collaboration (blogs, forums, IRC channels, mailing-lists and wikis) and Tracking Systems (e.g. bugzilla, GNATS, OTRS, trac). Despite going through them all can be time-consuming, those are the primary source of information to know more about an open source project.