Ahhhhh! Can't make just one!
Monstrous Regiment - Great tale of young girl who disguises her self as a boy to join the army, nice supporting roles by Sam Vimes and the watch.
http://www.amazon.co.uk/dp/0552149411/?tag=yah.007-21
Witches Abroad - Granny Weatherwax Rocks!
http://www.amazon.co.uk/dp/055215296X/?tag=yah.007-21
Going Postal - Fantastic story of reluctant heroes!
http://www.amazon.co.uk/dp/0552149438/?tag=yah.007-21
Thud! - A great take on Racism in Ank-Morpuk
http://www.amazon.co.uk/dp/0552152676/?tag=yah.007-21
Jingo - A great tale with capt Vimes taking a lead role and plenty of wacky inventions by Leonardo de Quirm
http://www.amazon.co.uk/dp/0552154164/?tag=yah.007-21
Can't wait til the 24th Septemeber for the release of Making Money, already pre-ordered mine for delivery on day of publication! ;)
http://www.amazon.co.uk/dp/0385611013/?tag=yah.007-21
You should also check out the audio books, Tony Robinson (baldrick/ Time team) does a fantastic job narrating them!
http://www.amazon.co.uk/dp/0552153028/?tag=yah.007-21