While I don't think the mystery of this book is as good as the one in the first book, I do think that the way that it builds on the story improves the first book. 

The characterization and exploration of characters from book 1 is brilliant, showing us a different side to characters we only really saw before in the context of the Andie investigation. It makes them more real, three dimensional and nuanced. The same happens to pip; negative aspects of her character are amplified and shown in a new light. The ending is also devastating. If you've read the first book, you have to read this one too.