Most RPG books are useless. This is one of those.
In Neil Gaimen's "Sandman", there is a scene where Dream curses a character to have endless ideas. One of the points that Gaimen intends to make, and it's a good one, is that an idea is in itself not even worth a penny. It's easy to have ideas. If you have any creativity at all, you could easily spout out a million ideas. But ideas are in and of themselves worthless. What makes an idea valuable is the blood and sweat that goes into turning that concept into a useful product. Having an idea for a story is a starting place, but it's not even the first step down the journey of having a story. All the value is in that work to turn an idea into a story.
Which is why as a guy who GMs a lot, it frustrates me to no end when RPG publishers publish works that are all ideas and almost no substance as if all I really needed was inspiration. I have almost limitless inspiration; what I don't have is limitless time. When I buy a book it isn't for inspiration. It's because I want to pay for the hours of time it would have otherwise taken me to make something. But invariably when I do buy a book these days I find that the author did only the easy work of putting down a bunch of loosely connected ideas on paper, but has left all the hard work of making those ideas usable up to the purchaser. I read through the concepts and most of them are pretty good if obvious, but the impression I get is, "This will take a further 20 to 100 hours of work just to get these concepts fleshed out enough to start play."
"Dreadful Secrets of Candlewick Manor" is billed as a campaign setting for the RPG "Monsters and Other Childish Things". As I'll explain in my review of the RPG itself, the RPG "Monsters and Other Childish Things" has a system that I really admire. As a GM and often amateur game designer, game designs are something that I collect and study, but a game system without an example of play is pretty useless because an RPG is much more than just its rules. Rules influence the play of an RPG, but how you go about structuring the play of an RPG and thinking about how to play it - what's called in RPG theory the metagame - ends up influencing the game more. D&D famously includes very concrete examples of play in the form of "modules" or adventures that lay out the expectation of how the game is expected to progress. What's interesting is that regardless of the game system, if you write a D&D style adventure for it, it plays very much like D&D. The mere act of preparing to play like the game will be kicking doors down, killing monsters, and taking their stuff creates a game that will be familiar to a D&D player, while subverting those tropes lets you play a different game while not even changing the rules. And often you can create completely new games by plugging in new subsystems into an existing ruleset.
Because I loved the rules of "Monsters & Other Childish Things", I very much wanted to also own an example of play that would tell me how it could be played. But in this I have been frustrated, because instead of being an example of play for the game described in the rules of "Monsters & Other Childish Things", this is actually an example of play for a completely different game with a completely different metagame that just happens to share some but not all of the same rules. And honestly, it's a lot more obvious how you would go about playing the game that is described in this book than the game described in the rule book, but that still doesn't solve the problems I have with the rule book for "Monsters & Other Childish Things".
The game described is heavily inspired by works like 'The Gashlycrumb Tinies' or 'A Series of Unfortunate Events' or perhaps 'Umbrella Academy'. The hook might be, "Imagine you were Wednesday or Pugsley Adams but you had been unfortunately orphaned and didn't know who you were or where you came from, and you were in an orphanage that held other similarly lost souls somewhere in HP Lovecraft's Arkham Country." This is perfectly fine as a concept for a game. It's a good idea. But that's all this book is: the idea for such a game, and about 160 pages of similar 'obvious' ideas about such a setting which ought to immediately occur to anyone well read, without any of the hard work to actually put in any substance that would be needed if you were going to have stories here. You'd need to write another 160 pages of notes just to yourself have any idea what was really going on, and you might could start the game with only a loose notion of where it was going, but I could point you to any number of million dollar writing disasters that happened because the authors started the story without really working out where it was going - Lost, the 4400, the Star Wars sequels, Game of Thrones, Wheel of Time, the Sherlock TV series, etc. etc. Maybe you'll start well, but you'll never finish well with that sort of beginning.
The excuse RPG authors always have is that they want to leave it up to the GM to decide what they want the mysteries to be so that they can tell their own stories. But if I have to do that, I might as well really tell my own story using entirely my own material. I buy other people's material to save me the hard work of making up my own stories, not to give me "inspiration". The real truth is that by avoiding the effort of executing the story and filling out the conception, it can always exist in the vague space of it could be a good story without needing to find out if by hard work they could actually make it so.
It's not an entirely useless book. I've seen "sandboxes" that are much more devoid of starting content. It's not entirely a rowboat world where the PC's have to work hard to find anything to do. And in fact, the most detailed aspects of the book are really quite good. For example, a lot of the monsters in the setting are very creative and both well-conceived and well executed. I would say the 15 monsters are by far the best thing in the book, and almost make the book worth the price of buying it just for that. But in terms of the value I get from the book, it fulfills neither of the goals I had in purchasing it. It doesn't tell me how you'd actually play "Monsters & Other Childish Things" as presented in the rulebook in a way that would be both social and gameable, nor does it present me with a ready made set of adventures that I could whip out without preparation if I happened to have four or six misfit teens in the house with a desire to try RPing.
I don't entirely regret the purchase the way I often regret an RPG purchase, but neither am I happy with it.