This is not from Wizards of the Coast, nor is it an official rulebook
The publisher of this work is Media Lab Books, a MacMillan Publishers subsidiary, also using the name "Topix Media Lab".
You can see indications that this is not a licensed work from the fact that the title uses the term "Game Master", not "Dungeon Master", because the term "Dungeon Master" is intellectual property of Wizards of the Coast and would require a license.
However, that does not mean it is "invalid", only that it is not officially endorsed. Third party publishers have the right to publish works under the Open Gaming License. Even while this is not an official WotC product, it is part of a series that has relatively high sales ranks on Amazon for D&D, so it seems to be quite popular with many players.