[Mithadan]

It depends what you mean by "rare". Some volumes of HoME are (or were) out of print in hardback, at least in the U.S. If I recall, these included volumes 2, 3, 5, 6 and 9. The entire HoME series is available in softcover in the UK, making volumes 6-12 "rare" in the U.S. Some people, myself included, collect true first editions of HoME (a "true first" is first edition, first printing).

Any first edition hardback of the Hobbit or LoTR and earlier second editions of LoTR are, to some extent, rare. U.S. Ace Editions of LoTR (the pirated version) are collectible as are the early "psychedelic" cover Ballantine editions. Even the later Ballantine editions (with JRRT's own artwork) are hard to find in good condition.

A good one to look out for is the hardcover, slipcased edition of Pictures by Tolkien, which was in print for only a few years in the 1970s. Another is the hardcover version of The Road Goes Ever On.

Collecting Tolkien books is a very personal endeavor. You should do some research, figure out what you want to collect and go after it. I know one person who is trying to get all printings of the first US sofcover Hobbit. Most of these are not "rare" as in valuable. But he likes them. Truly rare Tolkien books such as first editions of the Hobbit (US) or the first 4 printings of the first edition in the UK tend to be very expensive unless you get extremely lucky. Remember, there is a big difference between rare and collectible. The latter is whatever you want to gather up.