Is it permissible to use free practical examples available on the Internet, which are provided for the purpose of knowledge and not for profit, in authoring a book that aims to benefit Muslims and achieve material gain?

There is no objection to downloading practical examples for your project from the Internet, so long as their owners have not reserved their rights. There is also no objection to earning profit as a result of this work, because a person takes possession of permissible things by acquiring them.