For anyone who doesn't know Hellboy - imagine Sam Spade as a demon superhero investigating supernatural threats H.P. Lovecraft might have written and you're somewhere close.
Admittedly this is not the best of the Hellboy stories - Mike Mignola admits in an afterword that he lost control of the plot (the comic originally appeared monthly) - and if I were only marking the story I'd probably have wanted to give it 3.5 stars. Nonetheless, it is definitely worth reading with the trademark Hellboy mix of humour, action and horror, and the noir art as ever is wonderful (pushing it from 3.5 to a deserved 4).
A word of warning: although this is marked as vol.2 of the Hellboy series, there are stories in vol.3 'The Chained Coffin and Others' which precede 'Wake the Devil' chronologically.