The screenplay included was based on the continuity script prepared for the movie's original cut. Editor Michael Patrick Hearn (who had previously become famous in Oz world for The Annotated Wizard of Oz and had also annotated Huckleberry Finn and A Christmas Carol, and was already at work on his yet to be published biography of L. Frank Baum) reduced it to what was in the finished film. Deleted pieces of dialogue and scenes showed up as footnotes and appendixes.
Hearn writes a foreword that discusses how the script came to be written. The rejected drafts are mentioned, but it details how the final script it mainly the work of Noel Langley, revised heavily by Florence Ryerson and Edgar Allan Woolf, revised by Langley again, and finally a few uncredited (but important) tweaks from John Lee Mahin.
Included is the script for the deleted "Rainbow Bridge" sequence which was an alternate version of occurrences in the Wicked Witch's castle, cutting in right after Dorothy sees the Wicked Witch in the crystal ball. The Witch puts Dorothy to work, and she sings "Over the Rainbow" as she cleans the Witch's castle as the Scarecrow, Tin Man and Lion come to her rescue. The Witch is inspired by the song to create a rainbow bridge leading from one tower to another that would support one for awhile before they fall to a gruesome death, which she tests with a Winkie guard. Dorothy's friends arrive and wait in the opposite tower, as Dorothy tries the bridge. The Ruby Slippers keep her safe, though, and so the chase we know from the finished film ensues.
Also included are additional lyrics that were never intended for the movie: additional opening lyrics for "Over the Rainbow," "Ding Dong! The Witch is Dead!," "Off To See The Wizard," "If I Only Had A...," "The Merry Old Land of Oz," and alternate lyrics for "The Jitterbug." A lot of those will be familiar with Oz audiophiles, but Hearn surprises with an additional British sheet music verse for "Over the Rainbow":
Once by a word only lightly spoken
All your dreams are broken
For a while,
Sadness comes and joy goes by;
But ev'ry tear like the rain descending
Finds a happy ending
In a smile,
Doubts and fears all fade and die
To the blue beyond the grey
Love again will find its way...
EDIT: I have been informed that the script included with the Ultimate Oz collection is the complete continuity script. The same script was reprinted in 1999 for the DVD Gift Set, and a "Loose Leaf" edition is listed on Amazon. Premiere Magazine reprinted the script, but it appears to be only the script of the finished film. Any edition published by Faber and Faber is simply a United Kingdom edition of the book I have detailed above with Hearn's work. (Thanks, Kurt Raymond.)