This is an interesting mess of a supplement. I'm hopefully going to be writing a more in-depth critique over the next few days, because I've got quite a lot to say about it, but here are some general points:

• Given that most of the introduction is centred around player consent and comfort, returning to that theme at the start of several chapters feels redundant. I'm glad this stuff's in here, and I'd definitely rather have too much than too little, but it comes back a lot.
• Somewhat related, and something of a nitpick: why is there a Dresden Files Accelerated page reference every time conditions come up, even though the introduction specifically notes that condition rules are available elsewhere (including in this very book)?
• The use of conditions in general is more often questionable than fully justified. They often come across as a much more complicated way of doing something that could just as easily be achieved with consequences, stunts, or some other existing feature.
• The book generally takes too long to say what it needs to. I'm not sure we really need, for example, a full six pages describing how to give a monster aspects and skills, which are already the heart and soul of the system. I don't have a problem with advice as opposed to rules content, but a concerning amount of this book isn't really either of the above.
• A particularly egregious example of this problem: four pages on gore. It's pretty much just "use aspects or the Bronze Rule, stunts can make people resistant because stunts can do things like that", but in far, far more words.
• Consumable aspects are fantastic. The whole survival framework is smart and well-structured, and it's easily the best chapter on offer; my only complaint is that survival games feel like they might deserve more space than a single chapter.
• The feminine horror chapter feels like a flabbier, less focused take on Sarah Richardson's excellent piece in the Fate Codex (which makes sense since she has a writing credit for this). The connection to femininity for a lot of the listed themes is tenuous or unclear, and nowhere is this more blatant than with feminine horror aspects: they don't feel distinctly feminine, and, worse, the supercharged fate points they grant completely undermine the sense of helplessness they're meant to evoke.
• The Scooby-Doo chapter (sorry, I can't think of it as anything else) is fun and looks mostly sound, but a framework aimed at children feels very out of place in a book with this title, this cover art, and the dark, mature material featured elsewhere. There's nothing wrong with it but I'm not sure it belongs here.

All in all, the Horror Toolkit is a solid attempt to justify Fate for a genre it's arguably not all that suited to. I'm not sure I'd recommend it, but it's definitely an interesting piece, and horror enthusiasts may find some broad ideas and advice worth using even if the specifics aren't always well-executed.