Short answers, pulled from the story.
The Explorer's Guide to Wildemount was released on the 17th of March 2020. This date marked the official publication of the first campaign setting book derived directly from a fan-created live play series.
Matthew Mercer created the continent of Wildemount over years of live-streamed storytelling before Wizards of the Coast agreed to publish it. The development process involved rigorous playtesting and official vetting by lead rules designer Jeremy Crawford.
The four distinct regions of Wildemount are the Dwendalian Empire, Xhorhas, the Menagerie Coast, and the Greying Wildlands. The frozen continent of Eiselcross lies even further north beyond these four regions.
The Explorer's Guide to Wildemount introduced four new player races including the Pallid Elves, Lotusden Halflings, Draconblood Dragonborns, and Ravenite Dragonborns. These new options were accompanied by the reprinting of twelve existing races such as the Aarakocra, Aasimar, and Tortles.
Dunamancy is a new school of magic in the Explorer's Guide to Wildemount that manipulates time, gravity, and fate. This magic system included fifteen new spells such as Magnify Gravity and Gravity Sinkhole.
A leak on the 12th of January 2020 caused the book to rocket to the third spot on Amazon's best sellers list and reach the number one spot by Monday afternoon. It became the best-selling book on Amazon through pre-orders alone, outpacing New York Times best sellers and all other books.