It's kind of obscure. A lot of assumptions about the reader's knowledge level. Also about the reader's easy access to sources of animal parts. Sherlock reads along upside-down and seems amused.
Once Promise has a general idea of what the recipes look like she skims for the results of each without bothering about how many porcupines she would have to deprive of their quills or how much marrow she would have to scrape out of a live sheep.
Potion recipes make claims to such effects as: curse removal, securing the love of the drinker for the brewer (for a duration of one month), multiplying any nonmagical liquid to fill its container (size of container unspecified but from context they seem to be thinking flagons or barrels rather than lakes or ponds), and a dozen varieties of curse including warts, plague, petrification, loss of assorted facial features, and 'to Burn with an Unquenchable Flame'.
"Except for removing a curse if someone used one of the other recipes, I can't imagine wanting any of them. Maybe mortals have discovered extremely useful liquids of some kind that need multiplication."
"I can think of a few. If I were somewhere that animal blood was very scarce, I might like to have a flask of liquid-multiplier on hand. But yeah, a lot of old magic is primarily concerned with doing nasty things to one's enemies."
Promise puts that book down and opens the one about dark rituals.
There's a densely-written page and a half of advice on ensuring that you can't be easily traced: don't let them see your face; don't let them hear your name (where 'yours' is any you commonly use); keep very good track of your hair, fingernail clippings, blood, etc.; do not lose any items of sentimental value near a dark ritual site; if you suspect you may have lost a glove, sock, shoe, or other paired garment near a dark ritual site, burn or abandon the remaining item immediately.
"Very thorough," comments Sherlock regarding these safety measures.
"It's good advice, if you ever plan to get on the wrong side of the sort of person who does dark rituals."
"And I guess for this purpose 'Promise' in fact counts as my name."
"Yeah. If there's any fairies you'd like to curse with unspeakable harms, and you know what they call themselves and can draw a reasonably accurate picture of them, I'm sure I can come up with a book on how to do dark rituals."
"I'm trying to figure out what would happen to the court structure if I incapacitated Thorn. I don't know if Blossom could hold it together, how it would fracture if she couldn't, or whether she'd be any better than him if she could."
He shrugs. "Can't help you there. ...Although if you want to spy on them, this is also doable with magic and the requisite books might even be in this library somewhere."
"...Might help. But only so much. I didn't have a privileged view of the social relationships but I saw plenty and I think Blossom's basically brainwashed and don't know how she'd jump without him. Maybe Rainfall - I don't know."
They catch fire in sunlight. Certain items and substances (crosses, holy water) cause an instinctive aversion and raise painful blisters on contact. They are physically unable to enter a private home without the explicit invitation of a resident. They cannot eat food and must drink blood to survive. The process of turning a person into a vampire replaces their soul with some sort of demonic entity, which is why they are all irretrievably evil.