This book focuses on open source 3D printers. Open source is a philosophy, and does not refer to any particular type of design. Being part of an open source development community means that you use software or hardware that others have developed and posted online for public use, and that you in turn agree to share what you develop based on those earlier designs. In this way, very complex systems can be developed very rapidly.
But why would anyone post their hardware designs or software as open source? First, it is a very efficient way for technology communities to experiment and find good ways through a lot of possible growth ...