The Book Of Blood And Shadow

How could four hundred year old letters and a book have anything to do with the mysterious and brutal death of a modern day teenager?  The Book Of Blood And Shadow by Robin Wasserman will give you a pretty good idea.
Nora and her friend Chris signed up to help an old man translate an old Latin manuscripts and try to uncode a book that's hidden its secrets for four hundred years. Then Nora, in the Elizabeth letters she's been given, thinks she might have found an important clue- but she doesn't tell anyone.  Instead she takes the letter, wishing to keep it a secret for the four hundred year old woman who wrote them.  When she finally confesses to Chris, he promises to take them back for her.  She never saw him alive after that.
Arriving at his house for movies late, she expects Chris, Adrienne, and her boyfriend Max to have started without her.  But when she walks in she sees that her best friend has been brutally murdered, his girlfriend left with partial amnesia, and her boyfriend missing... and the police think he's the murderer.
Adrienne and Nora start out only trying to clear Max's name, but get caught up in a search for a four hundred year old machine.  The only problem is they're fighting time- and for their lives.

I recommend you read this book.  It was a great read and the ending was so unexpected.  The book itself kept me hooked the entire time and I begrudgingly put it down.