A good biographer, like a good detective, is a seeker of the truth.

But rather than seeking the truth of an unsolved crime, they seek the truth of their subject, however dark that may be. Sometimes, though, truth - emotional truth, particularly - can prove elusive. Such is the case with Dashiell Hammett, a man whose life is easily traced but not so easily interpreted.

Hammett was, according to this latest biography by Sally Cline, a man of contradictions. Like many noir and hardboiled protagonists who might have featured in his writing, Dashiell Hammett (Dash to those close to him) was not easy to like, but he was endlessly fascinating. A former Pinkerton detective agent, born in 1884, he turned to writing after ill health struck one too many times. He published five hardboiled crime novels over a relatively short period of time, but spent the rest of his life in a state of frustrated writer's block, while his long-term lover outshone him with her literary achievements.

He remained a distant husband to his first wife, Josephine, while living a separate existence- for 30 years - with the playwright and author Lillian Hellman, who loved and occasionally disliked this strange, angry man whose words would echo long after his death in 1961.

Dash would never finish that final book, but his five early works would ensure his place in the pantheon of writers who rewrote the rules of their chosen genre.

Cline takes a chronological approach to Hammett's life. The lengthy acknowledgements indicate that she has tried to gain a more intimate understanding of Hammett than others have before - this is less about the literature, and more about the man. In preparing for the book, Cline spoke to Hammett's daughter and granddaughter, as well as Peter Feibleman, Lillian Hellman's "heir and companion".

The book makes use of material previously suppressed by Hellman, giving us a darker picture of Hammett. His mood swings could be frightening, and nowhere is this clearer than in his treatment of Hellman during their Hollywood days. He beat her on at least one occasion and persuaded her to engage in threesomes with the Hollywood "chippies" that he chased. And yet his letters - there are many excerpts throughout the book - show a contradictory character. The missives from the war years to Hellman are touching, and Hammett's communications with his abandoned wife and family read as though written by a man who is on an extended business trip and missing those he loves.

For readers who know Hammett only from his writing, it is interesting to see parallels slip, often unintentionally, into his work. Nick and Nora Charles from The Thin Man clearly represent an idealised version of "Dash and Lilly". His early books include absent fathers, reflecting his own eternally drunk and violent father. The character of Sam Spade, from The Maltese Falcon, made such a lasting impact after only one novel and a handful of short stories because he was essentially unknowable and unpredictable. Much like Hammett himself.

Cline guides us through Hammett's life with little overt commentary or judgement. Like a writer of good hardboiled fiction - perhaps like Hammett himself - she leaves the reader to make their own moral judgements, allowing them to join her, the detective, in her inquiry. But this lack of analysis can mean the book feels a little light. At around 200 pages (excluding endnotes) the book is cut back to the bone. Sometimes facts might pass the careless reader by.

The impact of Hammett's treatment of Hellman is shocking in the brevity of its discussion, and we are left wanting more. Cline gives us the facts and a few interpretations as well as her own discoveries, which confirm Hammett and Hellman's lives to be as messy as one might already suspect. But, as with a good noir tale, it is ultimately up to the reader to uncover their own truths.

Hammett kept himself unknowable. But it seems clear his writing gave him the chance to be the kind of man he wanted to be. Physically, he could never be Hemingway or Faulkner, but on the page he was their equal in masculinity.

In her quest to illuminate something of the real Hammett, Cline sifts through fact and fiction (Hammett played fast and loose with facts about his life), ferreting out details from those who knew and loved him. And she discovers, in this highly readable biography, that Hammett was no hero - even if on occasion, particularly in his political beliefs, he tried to be. He was a gifted man plagued by darkly human flaws. Exactly the kind of person he might have been tempted to write about.