It seems from the descriptions that this not-quite-Fiftieth-Anniversary edition, like similar expanded editions of FGH, SWM, and OFS, includes the entire text of the original book plus ancillary material of great interest: the earlier versions of the poems where these are known to exist, such as "Looney" (better known as "The Sea-Bell") and "Firiel" (which became "The Last Ship"), not to mention the newly rediscovered original of "Shadow-bride". Of particular interest is the never-before-published "The Bumpus", which developed into "Perry-the-Winkle".
Best of all, Wayne & Christina print for the first time the prose fragment of what wd have been The Story of Tom Bombadil, had Tolkien continued it -- another of those "promising beginnings" as Tolkien himself called them that faltered after a few pages, like the sequel to FARMER GILES (similarly printed for the first time by Wayne & Christina in their extended edition of FGH).
Finally, this expanded edition of ATB adds the third Bombadil poem, "Once Upon a Time", a delightful little piece which seems to have been written just too late for inclusion in the original 1962 edition and instead appeared in print elsewhere a few years later.** So far as I can tell, they've not included associated poems like "The Dragon's Visit" and "Kortirion Among the Trees", which Tolkien considered including in the 1962 volume but which were ultimately left out for one reason or another (presumably finding it too hard to reconcile them to the 'Red Book' conceit that unifies the otherwise disparate collection). This was particularly unfortunately in the case of "The Dragon's Visit", which is a good deal better than several of the poems which made it in (e.g., "Bombadil Goes Boating", "Princess Mee", "The Man in the Moon Came Down Too Soon").
One thing I'm very curious to find out, and forgot to ask when I saw them this summer, is whether they've restored the original sequence of the poems Tolkien intended, or kept the revised sequence introduced by his publishers (w. Tolkien's permission) in the second printing (which is after all now of some fifty years' standing). In the original sequence, "Cat" is poem number eleven and "Fastitocalon" is poem number twelve; in the revised sequence, they switch places, so that "Fastitocalon" becomes poem number eleven and now precedes "Cat", which becomes poem number twelve.***
The chief significance of this is that Tolkien when refers in his Preface to poem number twelve . . .
No. 12 is also marked SG [=Sam Gamgee],
though at most Sam can only have touched
up an older piece of the comic bestiary lore
of which Hobbits appear to have been fond"
. . . he is referring NOT to "Cat" (the current poem number twelve) but to "Fastitocalon" (the original poem number twelve).
If they have not restored Tolkien's original ordering of the poems, then I'll be curious to see if they've changed the faulty reference in Tolkien's Preface, so that instead of "No. 12" it wd instead read "No. 11". And, though this is a lesser point, whether they've been able to restore the spot of color to the illustration of "Fastitocalon" (the tongues of flame from the fatal campfire) which was the root cause of the re-sequencing in the first place. We'll soon see.
Since the book itself's better than any description of the book, here's a link to what seems to be the amazon (.co.uk) listing for the expanded edition of this appealing little book.
--John R, looking forward to the arrival of my just-ordered copy
current reading: still the Echo-Hawk (only forty pages to go!)
**WINTER'S TALES FOR CHILDREN, ed. Carolyn Hillier , along with the revised version of "The Dragon's Visit"; both poems were reprinted a few years later in mass-market paperback by Lin Carter in THE YOUNG MAGICIANS , one of the sixty-five books in Ballantine's Adult Fantasy Series.
***for the reasons why this occurred -- a discovery that I made, ironically enough, when examining Christina's copy of the first printing -- see my article, written in collaboration with Wayne, "'Fastitocalon' and 'Cat': A Problem in Sequencing", which appeared in the August 1987 issue of BEYOND BREE.