The book «Using the HTML5 API by Eric Bidelman filesystem» is basically a deep guide to the new API included in HTML5 for file system management. This API provide to browsers the ability to access persistent data off-line, which is a breakthrough for web applications as it covers a major shortcoming for such applications.
The author covers every aspect of the API exhaustively describing each function provided by the API, covers topics such as file and directory management, security considerations and quotas, storage types and use cases among others.
I think it is a good book that provides to the reader a great introduction to the API but you have to remember that at this moment is only supported by Chrome browser and some functions and methods explained in the book are named with vendor specifics suffixes.
I can tell you as a web application developer that it is an interesting and easy reading book , it allowed me to improve my HTML5 knowledge and learn new and useful features i can apply in my daily work.
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