It's a digitally published book by Wizards of the Coast
You can read the description of this playtest material on Dndbeyond and via DMsguild.
It is described as:
Eberron was born in 2002 when Wizards of the Coast launched a worldwide search for a new campaign setting.
Wayfinder's Guide to Eberron brings the eponymous campaign setting to the fifth edition of Dungeons & Dragons. This 175 page book gives you everything you need to create an Eberron story or character, exploring the core themes of the setting and the rules to implement them in fifth edition. It includes an overview of Khorvaire and the fantastic city of Sharn, along with a host of background ideas and story hooks.
Playtest material includes the unique races of Eberron, the mystical dragonmarks (including greater dragonmarks and aberrant dragonmarks), and new magic items; this is a living document, and this content will evolve and be updated in response to feedback.
Keith Baker also discusses it on his site.
The introduction to the book also makes clear it's a work in progress for purposes of being a living document to help guide and inspire you and not be prescriptive rules:
Bear in mind: this book presents my current vision of Eberron. This is the world I run at my own table, and the way that I’ve converted its ideas to fifth edition. The game mechanics are usable in your campaign, but at this time they aren’t officially part of the game and aren’t permitted in D&D Adventurers League events. These mechanics will evolve based on player feedback. This is a living document, and as these concepts are refined, this book will also be updated for free; so you will be kept up to date with any changes that are made to it. Beyond that: Eberron is your world as much as it is mine. I hope that this book will provide you with inspiration, but don’t be limited by these ideas or decisions. Think of this as something you can build upon, not a codex of absolute law.