His correspondence with his brother.  The difficult question for those who take Heidegger seriously as a philosopher is whether, and how, the vulgar Catholic prejudices of his upbringing that led him down the road of Nazism and anti-Semitism can be disentangled from his philosophical views:  his hostility to philosophy, towards reason, towards modernity, and so on.  As George Steiner and Walter Kaufmann, among others, have argued, it is not entirely obvious that they are wholly separable.