Loved loved loved it!!
If you are a fan of 'From the Mixed-Up Files of Mrs. Basil E. Frankweiler' I can't recommend this book highly enough. Same sort of feeling with having to find out a mystery surrounding a piece of art.
I was just going to read a few sentences to get a feel for what the book was about when my day ended on Tuesday (at 1pm) because I'm in the middle of a book club book but I got so sucked in that I finished the whole thing in one big gulp and was finished by 4pm that afternoon.