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Mastering Ethereum is released under the Creative Commons CC-BY-NC-ND license, which allows sharing the source code for personal use only. You may read this book for free. You may not create derivatives (such as PDF copies), or distribute the book commercially. The full terms of the license can be found here:
Mastering Ethereum by The Ethereum Book LLC and Gavin Wood is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
It is expected that the book will be released under a more permissive CC-BY-SA license within a year of publication.