This book is developed like a free software project, your input and help are welcome. The most obvious way to contribute is to help translate it into your native language. But that is not the only possibility. You can open bug reports to let us know of mistakes, typos, outdated information, or topics that we should really cover. Or you can submit a merge request with your fix for whatever issue that you identified.
All the instructions to contribute to the book are documented on the book's website: