Title says it all - clearly The Hobbit, Lord of the Rings and the Silmirillian are there, but what other books should adorn the shelves?
Are particular editions ‘better’ than others? (Not so much for collectibility, but are certain revisions more important than others - if they exist)?
What about for Collectability? I recently aquired the Red Leather volume from 73 and a true first of the US silmirillian hardcover - what else should I start looking for?
For a better understanding of the stories, “Letters” is essential. He gives reasons for many story elements and a few fascinating what-ifs.
The volumes of The History are fascinating for lovers of minutiae and those who want to see Tolkien’s works in construction, but they’re not essential if you aren’t one of those people. Letters is important, as rowrrbazzle noted. For a non-Tolkien book that can give some geographical insight into Middle-Earth, I’d recommend finding a copy of Karen Wynn Fonstad’s Atlas of Middle-Earth. The maps in the published editions of the trilogy and the Silmarillion are unwieldy and not terribly naturalistic, whereas Fonstad’s maps tend to be more carefully drawn and she’s an incredible cartographer, so her maps are very carefully faithful to the text, when that’s possible.
Unfinished Tales gives some great tidbits and fragments which you won’t find anywhere else. As the title suggests, none of what’s in it is finished, but most of what there is is of very high quality.
The recently-released Children of Hurin contains a novel-length treatment of the Tale of Turin Turambar. It is, in places, some of the best writing Tolkien has ever produced, if the subject matter doesn’t drive you to suicide. It overlaps with Unfinished, in that parts of it were published there before Christopher was able to scrounge up enough old manuscripts to assemble the full thing.
I strongly recommend obtaining two older paperbacks of works by the Professor: Smith of Wooten Major/Farmer Giles of Ham, and The Tolkien Reader. They have some wonderful stories and poems that show that he was a very good writer, beyond just his ability to write about the mythology of his “Middle-Earth.”
You should, of course, obtain both the excellent biography of Tolkien written by Humphrey Carpenter, and The Letters of J. R. R. Tolkien, published by the combined efforts of Carpenter and Tolkien’s son, Christopher.
I would say Unfinished Tales and Letters are great additions.
I would also add the often overlooked novella masterpiece Smith of Wootton Major
As you specifically mentioned “what other books should adorn the shelves?”
I would say the Red Leather single volume of the Lord of the Rings.
The one I need to pick up soon is The Lord of the Rings: A Reader’s Companion ISBN: 0618642676.
I think at least one Hard Cover of the Hobbit should adorn a book shelf.
Jim
Another enthusiastic vote for Unfinished Tales and Letters, which are treasure-troves of Tolkien lore and inside stuff. I’d also add The Annotated Hobbit, which has a lot of cool stuff, incl. the derivations of all of Bilbo’s and Gollum’s riddles, the German postcard which inspired JRRT to create Gandalf, Gandalf’s post-Sauron explanation of how he selected Biblo to be the burglar, etc. Well worth a read.
One of my favorite Tolkien works is Tree & Leaf, collected in The Tolkien Reader. The first part is non-fiction, being Tolkien’s explanation & defense of Fantasy; the second part is an odd little story which feels like, but may not be, allegory.
Smith of Wootan Major, and Farmer Giles of Ham are must haves IMO. There is a book that contains both of these stories, with wondrous illustations, I think Hildebrandt did them? Not certain, and cannot dig up the book right now. ETA: What Exit?, Great Minds think alike, eh? 
Others have mentioned them already, but I’ll give my “essential” list. (Essential for knowing 90% of Middle Earth. I leave out JRRT’s other works in other fields)
The Hobbit
LOTR
The Silmarillion
Unfinished Tales
The Children of Hurin
The Atlas of Middle Earth
If you’re a real minimalist, skip Unfinished Tales and The Children of Hurin, but not the Atlas!
HOMES is for the Uber-student of Middle-Earth & JRRT. As are most of the analyses, commentaries, and essays on JRRT. Shippey writes good stuff.
Avoid “Middle Earth for Dummies”. It gets too much just slightly wrong. And it’s poorly organized.
J.R.R. Tolkien Companion and Guide is lovely for the enthusiastic student who’s considering becoming waaaay too into the topic.
Just IMHO.
I get this warm, fuzzy feeling knowing that you share my opinion of the Fonstad. 
Although JRRT didn’t write it, I’ve found The Complete Guide to Middle-Earth by Robert Foster (Del Rey Books, 1971, 2001) to be an invaluable resource. It’s got just about everything Tolkienesque that you’ll ever need to look up - alphabetically, concisely and accurately described.
Thanks for all of the hints - I have a tendency to become a completest - but knowing where to start makes SimWife much happier…
… quickly goes to revise the Christmas wish list…
Oh yeah. The Atlas is what first made Silmarillion really accessible for me. I’d read Sil with the Atlas on my lap, and suddenly I ‘got’ it.
I just love her Atlas series, pretty much every one of them. The only one I currently own, though, is the Middle-Earth one. I still pull that out just to look at the maps and read it.
I’m partial to Mr. Bliss. Having a Girabbit for a pet was a secret quest of mine for much of my childhood.
I know I have the Middle-Earth one, I keep it in my office with one of every Tolkien book. I think I have the Atlas of Pern somewhere.
Besides, if you want a good “for dummies” guide to the books, there’s already a perfectly good one available online.
Here’s a bibliography for all of the Inklings:
http://www.mythsoc.org/inklings.html
You can use it as a source for good secondary material on Tolkien.
Another excellent online resource: the Encyclopedia of Arda.
