'Guards, guards' is definitely the start of a chain; you'll get more out of 'Jingo' if you've read 'guards' and 'men at arms' first. Similarly, you'll know Susan better if you do 'the Hogfather' before 'Thief of time'. 'Mort' is more or less a starting point, too; although DEATH is universal. 'Pyramids' and 'Small gods' are standalones, can be read anywhere, at any time (although I don't recommend in church, at a funeral; people get quite bolshy about you breaking into uncontrollable giggles. Actually, they look at you a bit strangely when it's only on a bus, but can't say anything there).
Which leaves 'the Truth', probably better after a few of the other books, to get you into the environment. But the Truth is you can read, and get pleasure out of any of them, in any order. One or two minor running jokes might get missed, or be already explained when you get to an earlier book, but it's not critical. Each volume stands alone, and if you don't know Vimes when you start 'the truth' you'll meet him, be introduced, and what he has become by then will not disturb you when you find what he was before…
Enjoy.
