An intelligent, honest, inventive young man has invented the steam engine, and Lord Vetinari has to figure out how to contain and/or exploit it. Naturally this means Moist von Lipwig has a new job--representing the city's modest, but ultimately controlling, ownership share in the new steam engine company. This also means negotiating rights of way for the railway system, as neither the young engineer, Dick Simnel, nor his older and cannier business partner, Sir Harry King, the poop magnate, are well equipped for negotiating for land rights in foreign areas. For instance, Quirm, where the only reasonable route means clearing out bandits and negotiating with goblins.
When Lord Vetinari insists that a line to Ubervalt must be built very, very quickly, it's a problem. When a major political crisis among the dwarves means that he needs it right now, it's a life-threatening emergency.
Many old friends from previous books make greater or lesser appearances along the way. Like a few other recent novels, the tone is a little bit darker than typical of earlier Pratchett books, but it's still Pratchett and it's still a lot of fun. Moist in particular finds himself having fun, and even Vimes and Vetinari manage to get out of the usual constraints of their offices briefly.
This would be a bad place to start reading the wonderful Discworld series, but if you've enjoyed the earlier ones, you will enjoy this one.