In the Preface to their long-awaited J.R.R. Tolkien Companion and Guide, Hammond and Scull write that the book "has been designed to serve as a reference of (at least) first resort", but I would daresay that in a great many cases, theirs will be not only the first but the *only* reference work needed. It's *that* exhaustive!
The first volume is mainly a chronology of Tolkien's life, with several satellite chapters detailing his published works, poems, art, a series of Tolkien's family trees, etc. To say that the chronology is detailed would be a considerable understatement. It's really an almost daily accounting of the events of Tolkien's life: essentially an 800-page biography of dates. Staggering!
The second volume, the Reader's Guide, is even larger at well over 1000 pages. This volume, meant to compliment the chronology (and vice versa) provides alphabetized entries for just about every person, place, and literary idea of importance to Tolkien studies. Many of these are in quite astonishing detail, and even the short ones are extremely valuable little gems. For example, in the entry for Jennie Grove, Hammond and Scull provide the basic facts, of course -- but they also point out where to find a photograph of her as well as a portrait of her drawn by Tolkien. These are fantastic kernels of information, and nowhere else are so many collected together in one place. Not only that, but the list of unpublished and archival sources Hammond and Scull consulted is very impressive indeed! Many details represented here have never been brought to light before.
Another excellent feature of the set is the common index; that is, a single index at the back of both volumes covers references *in* both volumes. This makes cross-referencing between the Reader's Guide and Chronology a snap. Both volumes also have extremely thorough bibliographies.
The price tag for the two-volume set is high, yes, but it is well worth it. NB: I'm speaking of the U.S. edition published by Houghton Mifflin *only*; I have not seen the British edition published by HarperCollins (however, I can say that I've been consistently disappointed by the production quality of most British books -- including HarperCollins' otherwise excellent extended edition of "Smith of Wootton Major" (ed. Verlyn Flieger). The books are sturdy and well made, bound in cloth, with sewn binding. The slipcase, also, is attractive, sturdy, and cloth-bound and will help to protect the books over time. The pages are a pleasant cream, with a nicely proportioned and very readable font, and with appropriate margins. The pages had to be a little thin to accommodate so many, but there is only minimal bleed-through.
The two volumes are a little unwieldy to read from -- but given their size, how could they not be? And reading them cover to cover will take a long time (perhaps all the way until John Rateliff's History of the Hobbit is published next November ;), but it's something I've been looking forward to for a long time now.