Script Alan Moore, art Dave Gibbons and John Higgins.
This book was published by DC Comics through Graphitti Designs in 1988. Note that numerous Internet sources reference this book as being printed in 1987, but there are two forewords written by the authors dated January 1988 inside, and my copy is a first print.
It is a faux leather hardcover reprint (with embossed smiley face on front) fitted in a black faux leather slipcase (probably priced 50$, no ISBN, 456 pages, 7 x 11 inches or 18cm x 28cm). I don't know what is the print run but some sources talk about 1000 copies. The book is printed on the same very thin low quality mat paper as the original comics (yellow pages) and with the same size (see below). Note that the bonus section is printed on a little more thicker mat white paper and there is a sewn in red ribbon bookmark.
Both second and third of cover are different illustrations. Each original comic book is reprinted here with its cover, back cover and additional contextual document (Under The Hood ..).
The end of the book featured 47 pages of extra material separated in two parts, 'minutes' (introduced by Alan Moore) and 'seconds' (post-face by Dave Gibbons). 'Minutes' started with Alan Moore's original proposal for the story, first with the Charlton characters, then with the Watchmen character revision illustrated by Dave Gibbons early visuals (21 pages overall).
The four following pages display excerpts form the original script alongside pencils thumbnails. Then 'minutes' ends with promotional artwork and cover sketches (2 pages).
'Seconds' is only 6 pages long displaying extra material like one of Zenda exclusive cover and portfolio. The books ends with a picture of the smiley face on Mars (©NASA).
Some Artist Proof edition including a signed bookplate have seen the light of days on the second market, :
Scott VanderPloeg gives more insight about this edition on his Comic Book Daily site here, and more pictures on Comicstrade there.