Microsoft is offering a free book that provides tips, tricks on each and every feature of Windows Vista. The PDF version of the guide can be downloaded from here and if you live in USA you can order a free paperback by filling this form. The paperback guide is absolutely free and all you need to pay is for the shipping.
Reading of this book is highly recommended as many sections of the book are contributed by the people who actually worked on the product. This is the definitive resource to learn about Windows Vista.