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Sometimes it's easier to flip through this little book than typing in a search box or going to the MDN. This book has helped me more than once. It actually does provide some decent information about each of the methods in the Canvas API.
Pros: - Handy Reference - Good Explanations
Cons: - B&W Visuals kind of hard to appreciate - I don't like some of it's syntax choices
I purchased the pocket reference as quick introduction to the html5 canvas. My only mistake was assuming that the canvas was a larger topic than the reference makes it seem. Hopefully, O'Reilly can develop a fully realized HTML5 pocket reference once the language is finalized. Other than that, the book does its job quite well.
Summary: The book provides examples to explain feature after feature of canvas, carefully mentioning the concepts behind the feature being explained (strokes, gradients, transparency, text, shadows, etc). This book will explain how things are done and after that, you'll be able to grab sample code from the web and actually be able to customize it to what you need.
The book contains two chapters that are completely different of each other. The first chapter explains how things work in canvas. For example, whatever you draw uses a style that is defined separately from the drawing code, similar to the separation of content and presentation in HTML/CSS. The way you draw graphics, fill them, color them, etc is different than traditional environments such as Java graphics. At the end of the first chapter, you'll know what can be done using canvas and how.
The second chapter is a reference of the canvas methods, which will be the details needed once you're actually coding. Fortunately, there are not that many methods in canvas. If unfamiliar with pocket reference books from O'Reilly, they are small, you can carry it in the back pocket of your jeans. If you do not wear jeans or shorts at work, then you may not be the intended audience for this book. I needed a book that explains how to use bitmap images and do image processing in them. The few pages in the book on those topics made the book worthwhile. I'm looking forward to see the Web when most sites use canvas (instead of Flash and the like) for simple graphic animations or just improved user interfaces. (Disclaimer: I was provided a hard-copy of the book from O'Reilly for review and returned it back).