A companion to the Mars series (Red Mars, Green Mars and Blue Mars by Kim Stanley Robinson. The Martians is a collection of short stories, novellas and poems that surround the events depicted in the main series. Parts of the book include "Michel in Antarctica", an alternate reality where the Mars project was cancelled. A very interesting section is "The Constitution of Mars", which was referred to in Green Mars. Also included are additions to the mythology of Mars created in the three books, with the little red people and Big Man making a welcome return.
The Martians is a great companion to the Mars series, but is almost completely useless if you didn't like the series. If you did, however, you'll really enjoy this wonderfully varied collection.