I read this because I was reading another book about the Dresden bombing, and the wiki page cited this book.
In some places the book was 2 stars, in some 4, so I'm averaging it out to a 3. Honestly the plot just dissipates in some places (unless that's deliberately supposed to represent the pointless quest / wild goose chase they were on), but the emotions, especially the ones from grandparents, and then the bit about the inmate writing a plea for release, were sometimes heart-rending. Though as a non-American I don't think I'll really be able to fully internalize 9/11, I can still try to imagine. The sequence on the Hiroshima bombing, however brief, was affecting. And the aforementioned Dresden reference also induced the desired horror.
The Black key was a red herring. The old-people-sex was, uhm, a little odd. The beautiful though sometimes disoriented language in the letters from the grandparents make up for the sometimes forced precociousness of Oskar, and the use of photographs that I felt bordered on trendy/gimmicky. But overall the book did get across that sense of people experiencing grief and loss then trying to pick up the pieces (reflected in digging up the metal objects in the park, and hunting for all the Blacks), which I think was worth a dip into.