'Out' was the definitive 'noir' portrait of Japanese middle-aged female and 'housewife' culture, and I think 'Gold Rush' is the definitive portrait of sociopathic Japanese youth (it rings much more true than Kirino's efforts in this direction, by the way). Both books are also much larger than what I've just written, they're new, dark, and deeply truthful perspectives on Japanese society in general.
'Gold Rush' (like 'Out') is also an incredibly good read. You really don't know what will happen in the end: there are 'outside' 'adult' forces pushing to cover up the crime, others pushing for a confession, and this incredibly angry, unpredictable, and volatile 14-year-old boy in the middle.