Addenda & Corrigenda
 

This is maybe a bit academic but I'm linking to it just in case anyone may find it useful.

http://bcn.net/~whammond/addenda.html

This is a link to the Addenda & Corrigenda section of Wayne Hammond & Christina Scull's site (writers/editors of LOTR A Reader's Companion, the 50th Anniversary edition of LotR & the forthcoming Tolkien Companion & Guide).

It basically does what it says on the tin - updates/corrects entries in the above volumes as well as in their revised index to the Letters.