Other reviewers have said it all far better than I can, but I had to add my endorsement - what a terrific book!
I was hooked from page 1, but don't be put off by how dark the first few pages are - it's an introduction to the very damaged man known only as 'D'. After that we meet Jack: an ordinary person who witnesses a murder and does the right, and brave, thing by agreeing to testify against the killers. That's how he first meets D, a professional hit-man, who has been blackmailed into assassinating him, but, for various reasons, doesn't. This is the start of D's transformation, redemption, journey back to humanity - whatever is best to call it, he's changed profoundly by his association with and, eventually, his love for, the not-so-ordinary Jack.
I did like that things weren't passively 'accepted' - Jack has a hard time dealing with the facts of D's profession, and doesn't avoid the issue: he wants to understand, even though he doesn't agree with what D has done. He wonders if his feelings for D are genuine, or simply due to their forced circumstances. They argue and bicker, but Jack is intelligent and mature - the way he deals with D's inability to even admit, never mind communicate, his feelings is impressive.
That's not to say it's not also an exciting thriller - car chases, fights, double crosses: there's a great plot in there as well. This was well paced, romantic, thrilling, completely engaging. I enjoyed it so much.
I've devoured the short stories on the website, and am really looking forward to the next book. This is a story about love and hope. I've lost count of how many times I've read it and, every time, it makes me feel better.