Re: When there's no more magic left it's time to stop.
Raising Steam is one of my favourites; I think it's easily one of his best. Just because the social parody is more pronounced and the one-liners less prevalent doesn't make it more laboured or any less of a book.
In later years Sir PTerry became more and more angry about social injustice and inequality, capitalism and the rape of former nationalised industries for profit. This was evident to anyone who met him at a signing or convention. This anger translated into his work; he used the Discworld to hold up a mirror to our own so we could see the absurdity of the theories that underpin the modern banking system (Making Money), the idiocy of privatised public services (the clacks in Going Postal), and the stupidity of religious and racial intolerance (Thud & Snuff).
Raising Steam was both a chance to bring the railway revolution to the Discworld (something he'd been wanting to do for years) and to take a satirical look at fundamentalism. As far as I'm concerned he did both with considerable panache.