I have it but haven't started reading it yet.
I suspect the answer to Tamar's question is "not much". Probably the same amount as, say, the Compleat Ankh-Morpork or Mrs Bradshaw's Handbook. Most of the work is done by the folks at the Discworld Emporium (Isobel, Bernard, Ian and Reb, plus extra artists they hire), and the content may have been discussed with Terry so he possibly contributed some ideas, but probably not very much writing. The Emporium folks have been working on the Atlas for at least 2 years (they started just after the Compleat Ankh-Morpork was done, but before it was released), so its contents were more than likely discussed with Terry.
I know they've decided to cancel the Discworld Bestiary (which was to be the next project after the Atlas) due to Terry's passing, so some of the artwork already done for the bestiary has made it onto Teatowels...