The book addresses "geeky" owners of Canon consumer cameras who would like to explore the possibilities of their cameras, and eventually tweak them to do things way beyond their original specifications, such as
RAW file (or DNG) support in addition to the camera's JPEG output
Manual control for aperture, shutter speed, and ISO
Expand shutter speeds way beyond the specification limits
The hacks are based on the Canon Hack Development Kit (CHDK), a free software maintained by a group of enthusiasts. Many scripts are already published on the web. The book teaches how to use existing scripts and how to write new ones.
Canon cameras, especially their consumer lines of PowerShot/IXUS cameras, allow the user to temporarily upload so called add-ons into the camera through the memory card. The next time the camara is switched on, the add-on is active, when the camera is switched off, the camera returns to its original state, so there is no risk of loosing the manufacturers warranty.