Jingo is earlier than The Truth, and is some of the biting satire that he's written, as is Small Gods. Feet of Clay isn't hilarious, but it's grown on me - it has a quiet meditation on so much hidden behind the silliness, that each time I read it I see it a bit more. Lords and Ladies is also the best meta-novel (novel about stories and myths) he's written, with Witches Abroad and it's mediation on fairy tales a close second. He's been doing the serious stuff for a while - alongside silly books (like most of the Rincewind ones).

The Last Hero is worth getting, especially if you can get it on sale (it shows up in remainder sales, and Amazon mikght have it cheap). The story is definitely short, but the images are worth spending time with and looking at again and again.