I'm embarassed to say that I prefer the TV adaptation of Slow Horses, and don't think viewers will get anything out of reading the original novel. Typically I expect that the novel is deeper, smarter, and more complex than a TV series. Unfortunately for us readers, the adaptation is very faithful to the source material while suffering the sin of not having Gary Oldman.

The TV series provides many more scenes of Jackson Lamb tearing it up, delivering hilarious verbal abuse that is part of the charm of Slow Horses. The showrunners gave Gary Oldman more material and as a result, the novel feels like it's holding out on us. There's also a sense of effortless gravitas in an actor of his skill, and so while the novel tries to surprise you with Jackson Lamb's hidden talent, viewers instinctively know that underneath the dirty jacket is a man to be reckoned with.

On the other hand, the novel is a bit more intellectually complex. It's shorter and more focused. The adaptation also suffers from watering down its themes with Hollywood-action and requirements to give all major actors/actresses a certain amount of screen time. These are all qualities that do credit to the original source material. Sadly, the downside is to not see Gary Oldman. And that's just not worth it.

Recommended with reservations -- seriously, just go watch the TV series.