You could write a book about a technology you don't know but the book would be of little use as yiu would perhaps not know certain things, such as best practice

You could write a book about a technology you don't know but the book would be of little use as yiu would perhaps not know certain things, such as best practices, features and bugs with the technology. You would also do a disservice to users who pick up your book hoping to learn from it. As an example Learn C the hard way was written by Zed A Shaw of Learn python the hard way fame. He did not know C before he started and the book is currently on the "to be avoided" list on this page about C books http://www.iso-9899.info/wiki/Books http://www.iso-9899.info/wiki/Books