I don't usually buy setting books. The Sword Coast Adventurer's Guide was informative, but it didn't do anything for me. Eberron is a decent setting, but I don't need a book about it. I'm sure Wildemount, Ravnica, and Theros are great settings, but I'm not interested in learning about them at the moment.
The truth is, when I DM, I prefer to make up my own worlds. I might set a campaign in the Forgotten Realms for ease of PC backgrounds, but rarely are my players going to be visiting official cities like Waterdeep or Baldur's Gate. I just don't have that much need for other people's lore. I'm not saying my ideas are better than WOTC's, I'm just saying that pre-established cities and kingdoms tend to get in the way of the story I want to tell.
That said, I wouldn't mind a "Campaign Setting Sampler" book, with overviews of the most popular settings. Maybe 10 pages each devoted to Forgotten Realms, Eberron, Ravenloft, Greyhawk, Dragonlance, Dark Sun, Spelljammer, and so on. It might be useful for deciding where to set my next campaign.
Well, "Van Richten's Guide to Ravenloft" may not be that book, but if you're into horror settings, it does a great job of getting you started. Nearly half the book is just descriptions of the various realms of Ravenloft. For the uninitiated, Ravenloft is not a cohesive world like Toril or Oerth. It's more like a collective of pocket dimensions, separated by the magical mists that keep its inhabitants prisoner. Some of these domains are as large as a continent, while others are as small as a single city.
It devotes 4-6 pages each to to 17 of the more well-known domains, with maps, notable locations, and in-depth descriptions of each domain's primary antagonist. Then it gives short summaries to another 22 domains. So if you want to run a horror campaign and need a place to start, or even just enjoy reading about horror settings, this book is a great resource.
So what else is in this book?
The introduction explains the concept of Ravenloft's Domains of Dread, and gives a few tips on running horror games. Next up, we have three new Lineages - The Dhampir, Hexblood, and Reborn. Unlike the standard races, these lineages can be switched to after 1st level, to represent things horrible things that have happened to your character. Maybe you were bitten by a vampire, maybe you were cursed by hags, maybe you died and were brought back "wrong"... or if you are 1st level, maybe you were just brought into this world in an atypical way.
The Hexblood didn't do much for me - I didn't care for the lore or the features. The Dhampir was kind of interesting, with a (somewhat underpowered) vampiric bite and the ability to climb walls. The Reborn has some features that make it hard to kill, which is obviously pretty useful.
Next it has a section on Dark Gifts. These options give a character both advantages and disadvantages. The book suggests having players select one at character creation, but they also work as plot rewards. These are not the same as the "Adventurer's League" Dark Gifts that characters might receive after being resurrected in Curse of Strahd. These are more in-depth, and they don't seem to be as detrimental to the character.
The Dark Gifts don't seem very balanced with each other, and some are very specific to Ravenloft campaigns, so make sure you know what you're doing if you use these. For example, the "Mist Walker" gift is designed for characters who want to travel through the mist between domains. Another one that stood out to me was "Deadly Touch" which gives your unarmed attacks some extra necrotic damage, with no downside. Well, no mechanical downside. Roleplay-wise, it means you'll never get to touch another character's skin without hurting them. But if I were playing a Monk with a vow of celibacy...
Next up we have two new spooky-themed subclasses. For the Bard, we have the College of Spirits, and for the Warlock, we have The Undead patron. I don't see myself using either one, but they do fit the book's flavor quite nicely.
The book also contains two backgrounds: Haunted One (which previously appeared in Curse of Strahd) and Investigator. It also has an expanded version of Curse of Strahd's Gothic Trinkets table.
There's a chapter on the Genres of Horror. It has an in-depth look at the following genres: Body Horror, Cosmic Horror, Dark Fantasy, Folk Horror, Ghost Stories, and Gothic Horror. For those genres it includes tips and tables for selecting the right plots, settings, and villains for the story. Then it has a more abbreviated look at four other genres: Disaster Horror, Occult Detective Stories, Psychological Horror, and Slasher Horror. This chapter is useful both for idea mining, and making sure your campaign keeps a consistent tone.
There's a section called "Travelers in the Mists" which describes some of the more notable characters and groups you might run into during your travels. This includes some minor retcons to Ezmerelda d'Avenir and the Vistani, in response to changing cultural attitudes. Of course, now the less enlightened players are blasting WOTC for being too "woke", so you can't please everyone.
Then we have a chapter on Horror Adventures - More tips for running horror, including pacing, atmosphere, and setting boundaries. Then we have a look at Tarokka decks and Spirit Boards, followed by a Horror Tookit - detailing rules for curses, fear, stress, and haunted traps. There's also a section on "Survivors", which are basically simple characters that the PCs can control during expository scenes.
Next there's a short horror-themed adventure, "The House of Lament". The PCs explore a haunted house, perform seances, and learn of the house's troubled history. This is one of those stories where the horrors start out small but build over time, as the evil entities slowly awaken and become aware of the PCs' explorations. And it is most definitely an adventure that should be run on Halloween, by candlelight, while spooky ambience MP3s play in the background.
And finally, we have "The Monsters of Ravenloft" - a mini bestiary with about 30 horror-themed monsters. This includes tips on describing horrific creatures, and coming up with disturbing tactics that will keep your players on their toes. I could be wrong, but some of these monsters look familiar, and might be republished from other modules. My favorite monster in the book is the "Unspeakable Horror", which is a very customizable, amorphous... thing that is sure to make the PCs jump when they glimpse it in the mists.
All-in-all, it's a pretty useful book if you like running horror campaigns. Despite the new lineages and subclasses, there's a lot more here for DMs than for players. There is some repeated content, especially if you own Curse of Strahd. But there's more than enough new content here to justify the purchase, in my opinion.