Unseen Academicals, the 37th Discworld novel is out this week in hardback. You can get it from Amazon UK for about £9 including postage.
Nation, which is another Pratchett novel but not from Discworld, is out in paperback. Get it in the supermarket for less than £4.
Thanks Eccles! 
Looking forward to those whenever they arrive in cheap paperbacks, heh.
I have "Unseen Academicals" in HB cos I prefer HB to PB and it is OK so far. As with a lot of pTP's books it takes a while to get going but it is on form so far.
Ron
Just got this as a birthday gift and I'm eager to get into it. I may be gone some time 
I've just finished reading Unseen Academicals and I think it was pretty good. I won't say anything about the plot (though they are not the kind of books where you can give away the ending anyway).
I think I'll give it a 9. I thoroughly enjoyed it, but some of them were definitely 10s and left you breathless and ready to read it again from the start.
I've never had a Pratchett novel that I didn't enjoy and if this is the last it will be a shame.
According to "L Space" there are potentially three more DW novels that are in the planning stage:
I Shall Wear Midnight
Scouting for Trolls
Raising Taxes
I've heard of Midnight being discussed before; I think it is planned to feature Granny Weatherwax.
Raising Taxes is an intended sequel to Making Money with Moist von Lipwig.
I really hope he goes ahead with them. I got the impression he wasn't going to write any more.