I came across
this site yesterday and I have found it really interesting. Apologies if someone else has already flagged it but I couldn't find any previous reference to it here when I searched.
The author has analysed all the musical themes in LotR, when and how they are used. He's also included samples of variants, with explanations of how the leitmotifs change to fit mood. The different analyses can be found by clicking the links on the left of the homepage.