The "How it works" pages (from the FLASH Brochure 2007) are very helpful to get an good overview on synchrotrons, FELs and converntional lasers, SASE principle, electron source, cavities, superconducting linear accelerator, and the physics behind.
The relatively novel technologies and the free-electron laser principle are here explained on a level which should be useful to graduate students if they want to understand things beyond pure excitement, but also to scientists who may become interested in using this new tool for their own research or for their teaching.