My Soul to Loose is the prequel, and can be downloaded for free at either Kindle, Sony Estore or directly from Harlequin Teen.
The books should be read in this order:
Free prequel: My Soul to Loose
Book one: My Soul to Take
Book Two: My Soul to Save
Check out these vidoes for a taster - They look chilling, right? Definately on my 'to read' list.