So i'm rating this product on the price point it gives. Obviously it could go more into the society of the Undercity, give maps, encounters, etc. But for it's price it's fantastic. You get thematic and importantly creative creatures that feel truly unique and terrifying.
But beyond being great it does really good world building. Like in this book it talks about the Golgari using the druidcraft cantrip to open and close walls of moss to control the flow of water. That is a really creative use of magic and adds a cool integration of mechanics and world building. It makes the game and the world feel one in the same which is really important for a flavor rich setting like Ravnica.
TL;DR - This book gives you so much amazing and really well made information for it's price, its a steal.