Terry Pratchett is a political satirist, with very British humour along the likes of Monty Python and Douglas Adams. I suspect you will very much like his work if you are in a more relaxed whimsical mood at the time. His later work is definately starting to suffer from the incessant pace he is pumping them out.
Basically there are a couple of sub-plots which wind through the whole series that you should at least try to read in order (although this is not compulsory):
(a) Rincewind, the worst wizzard in the multi-verse
(b) Ankh-Morpork, the Patrician and the City Watch
(c) the Witches of Lancre
(d) Death, and his grand-daughter Susan
If you only have a limited amount of time, try reading his supposedly unrelated novels to get a taste for his style. "Good Omens" is typically regarded as his best non-Discworld book, but the "Truckers", "Diggers" and "Wings" trilogy is also a very easy read (it is suitable for children too) but very much show his style and his political opinions about sheeple.
Some of the books which are almost completely stand-alone (although you may miss some of the gags from time to time) are:
- Small Gods
- Pyramids
- faust eric
- Moving Pictures
- Monstrous Regiment
And, though it is a breach of the time-line, I really think you will get a big kick out of reading "Jingo".