Shnitzy: I once again recommend starting with the first Watch book, which is, as OneOfMany remembered correctly (as opposed to me) Guards, Guards! It gives a very good background of the general plot that will be useful moving forward (besides or being a good one in and of itself).
OOM: Actually, I didn’t hear about the Guide on here- I was recommended it by a friend. I then recommended it to the friend who recommended me Discworld- one good turn deserves another :).