I think one thing I love about Rivers of London is that it isn't just a series of novels, it's a whole WORLD that he's already fully created and fleshed out. So there are graphic novels and short stories and novellas along with the main novels. There are times it feels like he's writing fanfic in his own universe, if that makes sense. Taking a small element and spinning it out into something bigger, while being careful not to impact his own continuity.

The Furthest Station and A Rare Book Of Cunning Device can fit in pretty much anywhere in canon. The first is definitely worth reading. The second is okay. The graphic novels are superb, especially if you like the minor characters, and highly recommended. They flesh out Nightingale well too, which I like.