Let me count the ways this book fails:
1. It's called Michaelangelo's Notebook, but Michaelangelo, and the page from said notebook, are dropped promptly from the book after the opening chapters. False advertising.
2. The main character sits on her ass for most of the book. Where's the car chases, the rushing from one scene to another, the DANGER? Apparently this author resents Dan Brown - well, the problem is you're a pale imitator.
3. The author's vaguely sexist approach to his female character - he introduces her while she is nude, describing her body in great detail. Later on he has her think "this doesn't happen to me, I'm a girl!" She also goes from sexually inexperienced to jumping in the sack with a guy she just met who's 20 years older than her.
4. The author's writing is very bland and generic, and therefore uninteresting. He often fails the "show, don't tell" rule. Finn is at the computer and he tells us what she finds instead of showing her finding the results.
5. Touching on 2, the lack of an actual plot. Finn and Valentine go through the book almost on cruise control. They go and ask somebody something, they find something out on a computer. Too much talking, too little action. So pedestrian. Dan Brown meanwhile has his hero go from one European city to another, being chased by bad guys. Formulaic, perhaps, but here Christopher fails to even live up to pure formula.
6. It's all uninteresting. Looted art, American Nazis, a mysterious child.... yet Christopher spends most of the time having his heroes find out about the origin of a business and connections between names and people. I understand from other reviewers that the mystery of the child is never fully explained - I had lost interest long before and commenced skimming. If so, it's completely unacceptable not to give a proper accounting of what little plot elements you have laid out for us.