J. Bosch. Design and Use of Software Architectures: Adopting and Evolving a Product Line Approach. Addison-Wesley, 2000. See http://www.cs.rug.nl/~bosch/
Hyperworx is a platform for developing domain-specific visual programming languages that can be usefully applied to the creation of software product lines. See author ChrisRussell's profile for additional information beyond that posted on the main project page.