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", and "5th Editon RPG" not "Dungeons & Dragons 5th Edition", because both terms "Dungeon Master" and "Dungeons & Dragons" are Product Identity owned by of Wizards of the Coast and would require a license. The open Gaming License states (emphasis mine):

The following items are designated Product Identity, as defined in Section 1(e) of the Open Game License Version 1.0a, and are subject to the conditions set forth in Section 7 of the OGL, and are not Open Content: Dungeons & Dragons, D&D, Player’s Handbook, Dungeon Master, Monster Manual, d20 System, Wizards of the Coast

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.

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" instead of "Dungeon Master", and "5th Edition RPG" not "Dungeons & Dragons 5th Edition" – because both terms "Dungeon Master" and "Dungeons & Dragons" are Product Identity owned by Wizards of the Coast, and would require a license. The Open Gaming License, or OGL, states (emphasis mine):

The following items are designated Product Identity, as defined in Section 1(e) of the Open Game License Version 1.0a, and are subject to the conditions set forth in Section 7 of the OGL, and are not Open Content: Dungeons & Dragons, D&D, Player’s Handbook, Dungeon Master, Monster Manual, d20 System, Wizards of the Coast

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.

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.

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", and "5th Editon RPG" not "Dungeons & Dragons 5th Edition", because both terms "Dungeon Master" and "Dungeons & Dragons" are Product Identity owned by of Wizards of the Coast and would require a license. The open Gaming License states (emphasis mine):

The following items are designated Product Identity, as defined in Section 1(e) of the Open Game License Version 1.0a, and are subject to the conditions set forth in Section 7 of the OGL, and are not Open Content: Dungeons & Dragons, D&D, Player’s Handbook, Dungeon Master, Monster Manual, d20 System, Wizards of the Coast

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.