This is the first part of the young adult series "The Mortal Instruments". The story takes place in today's world - it starts off as urban fantasy, but later will also move to more fairy-tale-like places.In the "Mortal Instruments" world, there exist demons, vampires, werewolves, fairies, witches and angels. The story is focused on shadowhunters (also called Nephilim), humanlike creatures that have some angels' blood in them. They're grown from a young age as warriors, mostly fighting with demons trying to break into the human world, but also keeping an eye on other supernatural creatures to keep them in line. The main character is Clary, an almost 16-year-old girl whose normal teenager's life is suddenly pierced by supernatural events. Being in a night club with her friend Simon, she sees three persons around her own age killing a fourth - and discovers that neither Simon nor anyone else sees these teenagers. The unknown persons claim that they had killed a demon. One of them - Jace - finds Clary on the next day and invites her to come with him, explaining that the fact that Clary saw the murder means that she isn't a regular human being. And so start the adventures that turn Clary's life upside down.Of course you shouldn't expect a deeply philosophical epic story from a young adult series, but as entertainment, this series is worth reading. Although the characters sometimes behave a little chaotically (but maybe that's how teenagers are? :P) and some "surprising turns" are relatively predictable (then again, some things caught me quite unexpectedly), the story is very enthralling, the books are hard to put down, and the knowledge of having an unfinished book near makes it hard to concentrate on something else. In this series, you'll find romance, excitement and mystery, some action and magic, colourfully described (sometimes rather funny) scary monsters and, of course, beautiful human characters.The least believable for me is that young, 15-18 years old shadowhunters are already well-trained warriors with many battles behind them, and that they can successfully fight against bad guys much older and more experienced than them. It suddenly seems really odd when, in the third book, before a large battle, most of the main characters are left home because they're "only children" - where were the parents, when these same teenagers were sent on dangerous missions to hunt demons? At least the author has tried explaining this - that the Nephilim simply need every available fighter, and that the main characters' parents have done the same thing when they were young. I guess this kind of thing is inevitable in young adult literature - the readers need someone of their own age to sympathise with.As a conclusion - despite some shortcomings, this is a nice and enterteining story which I would recommend to others who generally like young adult novels. I myself am already looking forward to the publication of the final installment of the series.PS. Next summer, this book can also be seen in the cinema! :)This review has also been published in Estonian on the blog Fantaasiajuttude muljed.