Kant on modality

Recently, I have been thinking a bit about Kant's position on modality. Since this topic is not much discussed and since I do not have very clear views on it, I would be grateful for reading suggestions relating to this issue (both primary and secondary sources). My current interest in modality is, on the one hand, due to the fact that over the course of the last three days I have been attending an impressive conference on the epistemology and metaphysics of modality that took place in St Andrews. On the other hand, Andrew Roche and I are still discussing Kant's account of freedom over at Kant blog and the discussion has recently turned to the modal conflict, i.e. the conflict between the contingency of spontaneous actions and the necessity of our actions that are determined by laws of nature. In particular, the issue is to what extent Kant's distinctive appeal to nested modalities requires a two-world interpretation of transcendental idealism.