The book covers the following topics:
• The key players in the history of contraptions
• The different mechanical components often found in contraptions
• Scientific, mechanical, and engineering principles involved
• How to build working contraptions of your own
Be warned: there aren't really 50 unique projects in this book...I would say it's more like a dozen. Also, these are contraptions in the Rube Goldberg tradition. Though the title insists they are "useful", they are not -- just silly, complicated, mechanical fun.
Here's where you can get Make 50 Wild and Wacky (but Useful!) Contraptions.