Tolkien: The Encyclopedia of Arda -- great for settling those insanely geeky Lord of The Rings arguments, of which there have been loads recently. ;)
For example, yes, Gandalf does wear one of the elf-rings -- and this is shown in the 'sailing of the ring-bearers' scene in the third movie. (The elf-rings are only intermittently visible in the movie -- presumably because only other ring-bearers can see them, or something like that. Ben?)
I'm looking forward to the extended edition of Return of the King already...