So Vile a Sin is the second book in the Peter Grant/Rivers of London series by Ben Aaronovitch.

The story opens with DC Peter Grant, a trainee wizard and police officer, getting called to investigate a murder. The victim, a wealthy businessman named Eric Odeh, is found riddled with bullets in his own car, which has been pushed into a canal. Along with his partner, DS Lesley May, Peter begins to investigate the case, which quickly becomes more complicated than they first anticipated.

As they delve deeper into the case, they discover that Eric Odeh was not all he seemed. He had ties to the criminal underworld and was involved in a range of illegal activities. Peter and Lesley soon find themselves in over their heads, as they try to navigate the corrupt world of organized crime and the seedy underbelly of London.

Things become further complicated when one of their witnesses, a notorious criminal named Walter Reynolds, goes missing. Walter is not just any criminal though, he is a wizard with a long history of involvement in magical crime. With the help of Nightingale, a senior wizard, and their own magical abilities, Peter and Lesley set out to find Walter and determine his connection to the murder.

In the meantime, Peter's ongoing struggle to balance his normal life with his wizarding duties becomes increasingly difficult. His girlfriend, Beverly Brook, a river goddess, is determined to take him deeper into the world of the rivers, while Nightingale is pushing him to take on more advanced magical training. Peter struggles to find a balance between his loyalties to the police and his duties as a wizard, while also dealing with personal dramas in his personal life.

As Peter and Lesley uncover more evidence, they begin to realise that the murder of Eric Odeh may have been part of a larger conspiracy involving the criminal underworld and an ancient magical order. This puts them both in danger and pushes them to their limits as they try to solve the case before it's too late.

In the end, Peter and Lesley discover that the real mastermind behind the murder is someone they least expected. As they confront the killer, they are forced to make a series of tough choices that will change their lives forever.

So Vile a Sin is a fast-paced, engaging mystery with plenty of twists and turns to keep readers on their toes. Peter Grant is a charming protagonist, balancing cockiness with a deep respect for the magical world he inhabits. Fans of the series will love this second instalment and newcomers will find plenty to love about Aaronovitch's unique take on the police procedural genre.