S.G. Maclean's The Seeker (Seeker Series Book 1)

London, 1654. 

London is a teeming warren of spies, exiles and assassins that is Cromwell's London, the Lord Protector's most feared agent is Damian Seeker. No one knows where Seeker comes from or even his real name. One thing is certain - nothing remains hidden from him for long. 

When a popular captain and hero of Cromwell's all-powerful army is murdered, and Elias Ellingworth, an outspoken critic of Cromwell's regime, is found standing over the bleeding body clutching a knife, his guilt seems to be without question. Yet Seeker is not convinced. He will stop at nothing to unmask the true killer and save Ellingworth from the gallows and knows better than any man where to search. 

Paperback: 398 pages.