At the time of writing version 4.7 is the current stable release and 4.6 (which we used for this book) is the Long Time Support (LTS) version. On average, CiviCRM releases two major releases a year, along with numerous minor revisions to deal with bug fixes. In general, the CiviCRM development cycle typically follows the open source mantra of release early, release often. There is also continuing discussion on the road forward; the latest vision is to first release new functionality as extensions.
The core team relies heavily on the CiviCRM blog to provide information to the community. Continue to watch the blog for more information.
