Of the three books, it is the weakest, in my opinion.
I was expecting something that was at least a travelogue, taking you to various locations across the map. Instead, it talks more about the individuals in each world, rather than about places. For example, while it mentions the Tomb of Horrors in the Greyhawk section, it focuses on Acerak, not the tomb.
Overall, it's light on details of what makes each world. The art does some nice sifting through the editions, but it not even a primer level of detail about the various worlds - the 2024 DMG's coverage of Greyhawk is way more in-depth than this book.