Short answers, pulled from the story.
Guildmasters' Guide to Ravnica is a 256-page campaign and adventure guide for the 5th edition of Dungeons & Dragons, built around the Ravnica setting from Magic: The Gathering. The setting features a single city spanning an entire planet, governed by ten competing guilds of different ideologies. The book provides racial options, subclasses, adventure hooks, and stat blocks for the setting's creatures and non-player characters.
Guildmasters' Guide to Ravnica was published on the 20th of November 2018. Its cover and full product listing were leaked on Amazon in July 2018, months before the official release. Cover art was provided by Magali Villeneuve.
James Wyatt was the lead designer of Guildmasters' Guide to Ravnica. Wyatt had worked on Dungeons & Dragons for over a decade before joining the Magic: The Gathering team in 2014. He described the book as "essentially, Plane Shift: Ravnica," a reference to his earlier series of free D&D adaptations of Magic settings.
Guildmasters' Guide to Ravnica introduces five new playable races: centaurs, minotaurs, loxodon, Vedalken, and Simic hybrids. The book also adds two new subclass options, the Order Domain Cleric and the Circle of Spores Druid, along with Ravnica-specific magic items and treasure.
Reception was mixed but largely positive. Publishers Weekly placed it at number seventeen on its Hardcover Nonfiction bestseller list for the week ending the 3rd of December 2018, calling it a "Black Friday winner." Reviewers praised the mechanical content and setting art, but some noted that running actual adventures there required significant preparation from the dungeon master.
Ravnica originated in Magic: The Gathering, first appearing in the card set Ravnica: City of Guilds, released in 2005. James Wyatt began adapting Magic settings for Dungeons & Dragons in 2016 through a series of free PDF documents called Plane Shift, covering six planes including Amonkhet, Innistrad, and Zendikar. Guildmasters' Guide to Ravnica was the first full hardcover D&D guide to a Magic: The Gathering setting.