This book was discussed on Mage the Podcast in this episode

TLDR: This is a good book that is a bit of mess with internal inconsistencies and vaguenesses that stop it from being truly great, but within its pages are the broad group of ideas that any Storyteller who wants to go beyond the Gauntlet will find endlessly useful. Consider getting Infinite Tapestry for a mechanically tighter presentation and a stronger focus on how to actually use the various areas.

This book provides Mage players with a torrent of information on what exists beyond the Gauntlet. Prior to now, some realms had been mentioned like the Maya being the land of warring Dreamlords, and the Middle Umbra having been described in 1993 in Werewolf but much of what Mage Mage was previously undescribed. This book came out of the gate with the High Umbra, the Vidare, the Shard and Shade Realms, locations beyond the Far Horizon, and Umbral, Horizon, and Paradox Realms largely alien to the other game lines. It was ambitious and it was messy.

The book leads by saying that the Umbra defies mapping. Narratively this may make sense but mechanically it presents complications. This could prove to be a get out of jail free card as it allows both the ST to ignore their own errors as well as allowing the authors to sweep their inconsistencies under the rug. While I understand that mapping could be folly in the traditional sense of looking for dimensions, a map also shows relationships. A subway or bus map can be stylized so long as it shows what’s at each stop as well as the relationships of one stop to another. I don’t much care how big the Legendary Realm is, but it’s nice to know roughly how to get there even if that’s not tied to spatial coordinates.

The page count the book dedicates to different areas feels highly hit-or-miss. Very little is given to the penumbra except in broad strokes and despite Vidare being nonsensical (if a character in the Vidare Mortuum gets on the Midnight Express and zooms away, what do the other characters see?) there’s no good explanation of why one would use the Penumbrae. The Vulgate and the inter-realm area of the High Umbra are also given little detail to the point where if I were to strictly use the book I don’t know if I’d be able to describe it. We get three or four locations in all of the Vulgate which is pretty light.

On the flip side, the Dream Realms are gone into with great detail to a level that I as an ST would never use. The Dream Realms aren’t given narrative justification to back the expansive page count for the Land of Nod and Hollywood. It has made me appreciate the focus in more recent books on “if it’s in here, it has to do something useful and that useful thing must be explained”.

Within a section, the results are varied and inconsistent. Some of the Shard and Shade Realms are discussed in exhaustive detail while others are barely considered. There’s also unclear information on what each place actually is with the Shard or Shade Realm possibly being the planet vs being the planet’s Penumbra vs something else entirely. I found it largely confusing. But what it does offer is a heaping helping of ideas. The book overflows with options for novel interactions and play, albeit with clunky mechanics and high barriers to entry. For reasons I don’t understand, most of the places the book says you can go require 4 or more dots in something from at least one member of a group which more or less makes this an intermediate or advanced area of play. Unlike the digital web which has VR as a quick way to enter it, there is really no analog to entering the other places.

This book feels very transitory between 1e and 2e. There are a lot of authors whose ideas are in this book and some are wonderfully fleshed out and others questionable. Many of the areas have little useful information but high detail. The Afterworlds are lengthy and seem to just be a space for introspection with little reason for a character to go there. Some of the difficulties presented are quite high, often 9 or higher which seems like a terrible narrative choice as I’d not want players interfacing with something where the odds of catastrophic failure are quite high. But along with 1e comes some imaginative ideas. The COP is outlined in detail making it a giant beautiful place and the contrast with the Darkside Moonbase is strong.

Book of Worlds has many things that I disagree with in it, some of which were changed by Infinite Tapestry, some of which I just think go against the themes of Mage that I prefer. Overall, though, it's a solid book, and has a lot to offer in its tour of the Otherworlds.

One thing I wasn't a fan of was the experiment of separating the mechanics from the flavor. Having the entire book written in character was a mixed bag, but worked for what they were trying to do. But not including sidebars with mechanics and relegating them to the appendix will make it a lot harder to find things in it later.