I have long been a fan of “How Stuff Works” (http://www.howstuffworks.com/). When the book (also named “How Stuff Works”, http://www.amazon.com/exec/obidos/ASIN/0764565184/benfortascoldfus/) came out I bought it immediately and was not disappointed. Marshall Brain (yes, his real name) and his team have mastered the art of taking all sorts of concepts and making them clear and simple and even fun. Both the site and the book have highly readable text, clear illustrations, concise and definite explanations, and set the bar for how information should be presented and taught.
I spent the past few days perusing the sequel, “More How Stuff Works” (http://www.amazon.com/exec/obidos/ASIN/076456711X/benfortascoldfus/). Lots of great new content on topics as diverse as hurricanes and tornadoes to PDAs to paintball to lie detectors to the Hubble Space Telescope to lock picks. Over 100 topics in eleven categories.
Production quality is superb, these look more like coffee table books than they do tutorials, and both are fun and highly recommended for kids and adults alike.