Is the “Immortals Handbook” series official WotC material?

Regardless of any thoughts on the general quality, as sources seem conflicted on that, my question only relates to how official these books are. How valid should I consider them? Are they actual WotC material? Are they acknowledged by WotC? Or are they wholly 3rd party? Also, for what edition are they intended anyway?

The series I am referring to includes this book.