This book received glowing praise in the press, and while I am not saying that it is in any way a bad book, it doesn't deserve the extravagant praise it has got. In particular, it says very little about the picture. You can find out more from Wikipedia. Nor does the author give any opinion on where she stands on the controversies surrounding it. Her history of the picture's reception is the most interesting part of the book, but the problem is, most of the book is taken up with the lives and circumstances of the picture's owners. And when this descends into the dynastic politics of the 16th and 17th centuries, frankly you have to be pretty committed not to skip the numerous pages devoted to this. Even the description of the Peninsular War, which is somewhat more lively, is not exactly gripping.
I hope and presume the author wasn't responsible for the title. (She says herself that the woman -- and no way is she a 'girl' -- seems to be a standard 15th century portrait based on no one in particular; it's the man who's individual and interesting, and it's the symmetry that makes the picture.)