If you're looking for lots of Buffy the Vampire Slayer-type action with a teenage romantic triangle thrown in, the first book of the Mortal Instruments series has all of that. Demons, wizards, vampires and werewolves; all kinds of exciting supernatural characters and the shadowhunters who interact with them. Ordinary humans are called "mundanes" in this book. There are long-kept family secrets, dark betrayals, acts of courage and strong friendship loyalties.
Clary, apparently a mundane, isn't good at following instructions; the first thing she does when her mother tells her over the phone NOT to come home is to return to her apartment, where Clary is attacked by a demon. A shadowhunter named Jace comes to her rescue and then joins her in her quest to find her mother. Clary looks when she is told not to and stands still when she is instructed to run. Still, she discovers that she has unique, innate abilities and holds her own with her shadowhunter companions.
The writing style is not at all to my taste, relying heavily on cliches: raven black hair; blinding white table linens; frozen in horror; sat bolt upright (with a sudden realization of something that this reader had figured out 100 pages earlier); "dropped the truth on them with the weight of a crushing blow" and so on.
Sometimes the descriptors were oddly chosen, as when Isabelle was "holding a round spoon in her hand" -- since all spoons are rounded, why not describe it as wooden, or metal or long instead, and "in her hand" seems redundant; only mention if she was holding it in some other way, like in her mouth. The tarot cards were "tied up in a silk ribbon." There's nothing wrong with "silk" per se, but why not give us a colour? None of the characters are blind, after all. "White smoke" came out of the teapot when Clary sat down to have her fortune told. There was real tea in the pot, not incense. (I couldn't help but unfavourably compare this bit to the poetic writing by Sonya Harnett in The Ghost's Child where, when Matilda sets down a pot of tea, "quiffs of white steam waltzed and vanished.")
Other times, phrases did not make sense. Clary's arms were "aching and stinging like raw meat." They may have looked like raw meat after her fall, but does meat feel hurt? A "folded piece of paper fluttered to the floor." Clary picked it up and smoothed it out and discovered that it was a photograph. Unless they have been printed on regular bond paper, as on a computer printer, photographic paper is heavy and doesn't "flutter." This was a photo of Clary's parents and a circle of their friends, taken before she was born. There doesn't seem to be any problem of crease lines interfering with the clarity of the image.
Quibbles and overwrought prose aside, the plot kept me interested to the end. This would appeal to fans of Stephenie Meyer's Twilight, especially those readers looking for someone with a little more pluck than Bella.