Guards! Guards! by Terry Pratchett
When a disgruntled secret society wants to bring back the monarchy naturally they begin by summoning a dragon. Because, of course, everyone knows that a long lost heir to the throne will emerge to fight it. That should be enough to tell you that you are in Terry Pratchett's Discworld. We also meet the Watch members, charged with maintaining law and order, or at least a fiction of it. Quite enjoyable although indeniably an early Pratchett where he's not at the height of his story telling powers. His humor, though is, as always, right on target.
I just reread this because I wanted something easy for nighttime reading. It was more fun than I expected because after having read the entire City Watch subseries it reminded me of just how far Vimes and all the Watch had come since their humble beginnings in this book.
This is a good place to begin if you want to try Terry Pratchett and jump into Discworld.