Book: A Tolkien Bestiary by David Day, and various artists
If you get one "companion" publication to The Lord Of The Rings (or even The Hobbit), make this it. It is a source of great puzzlement to me as to why this has not been re-released in the wake of the films. Given the top quality of this book, it could only help the popularity of LOTR, and of Tolkien's work in general. The most recent version was published in 2002, but don't let that recentness fool you - a good few months ago, when I remembered having read a much older version (well, I looked at the pictures), I decided to try and seek out a copy. Amazon was my first port of call, but I waited for the 2002 version to be shipped...and waited, and waited. In the end, my order was cancelled by amazon, and so instead I turned to their zshops. I found (what i thought was) the 2002 version at a pretty cheap price (about £10/$15 including p+p) and apparently in good condition, and so I ordered. It arrived in as new condition and carefully padded against the rough postal service. Having just searched on amazon, it turns out it's actually the 1996 version - in total there have been nine versions published, according to amazon anyway (there could be more in fact).
SO why is this such a good book? quite simply, it consists of a good deal of short stories on the history of Middle Earth (going back alot further than the creation of The Ring), along with the shorter descriptions of places and peoples. There's also a large number of beautiful illustrations, a few of which are in colour and all of which are detailed (and having been contributed by a total of 11 artists). The history, although it does go into some detail, is like a potted version of all of Tolkien's Middle Earth stories bundled together, plus a bit more. The whole thing was inspired by the Bestiaries of old which, according to the preface, were "...after the Bible, the most widely read and disseminated written works in the West", and whether that is true or not, there' no denying this book is a work of art. Even those who are not fans of Tolkien's work will appreciate the artwork.
A word of advice if you decide to go get yourself a copy: if, as it appears, you can only get secondhand copies now, you really shouldn't pay more than about £10UK/$15US plus postage. It's not actually rare yet and there are many copies for sale out there, although it may take a little bit of searching to find a good condition one.
(The book appears to no longer be in print. I advise looking at amazon or ebay)