Agatha Christie’s Secret Notebooks (HarperCollins, £20; Buy this book; 492pp) weren’t really a secret, but they have had to wait for the meticulous attention, dedication and prodigious knowledge of John Curran to achieve publication. They’re not really diaries, containing no hint of her day-to-day activities or intimate thoughts, but 73 notebooks in which she wrote, in pencil, pithy ideas for plots and characters that often, but not always, developed into her novels and short stories. To make Curran’s job more difficult, Christie’s handwriting was often almost impossible to decipher, her entries were usually undated and the notebooks were in no chronological order. Ideas that she had decades apart were found on adjoining pages. There was nothing systematic, and not all her books are referred to — there is almost nothing about two of the most famous, The Murder of Roger Ackroyd and Murder on the Orient Express.

The contents of the notebooks are too haphazard to reveal much about Christie’s writing methods, though there is a lot of entertaining trivia to occupy aficionados (for instance, Death on the Nile was originally conceived as a Miss Marple mystery, Poirot being substituted because Christie felt he would be more at ease in a foreign setting). Curran’s commentaries are essential companions to Christie’s scribblings.

The book also contains two “hitherto unpublished” Poirot stories. One of them, “The Incident of the Dog’s Ball” is, to an extent, a prototype of the novel Dumb Witness, though with a different conclusion. “The Capture of Cerberus” — a completely different story from the one that appeared under that name in The Labours of Hercules — is odd and wholly unsatisfactory. Probably written in 1939, it features Poirot wrestling a ludicrous political conspiracy tale involving none other than Hitler, thinly disguised. No wonder it was not published at the time.

Ten Little Herrings (Macmillan, £16.99; Buy this book; 248pp) — red rather than fishy — is a delight. Most attempts at affectionate pastiches of Christie-like whodunnits disappoint either because the plot fails to match the humour, or the writer lacks the essential lightness of touch. L. C. Tyler succeeds on all fronts. His duo of inept accidental detectives, the crime author Ethelred Tressider and his literary agent Elsie Thirkettle, in their second outing, find themselves in a shabby French hotel at a convention of stamp collectors. Two murders occur. True to the genre, no one is allowed to leave the premises and many of the guests are not what they seem. Tyler juggles characters, story, wit and clever one-liners with perfect balance.

In Blood Line (Little, Brown, £16.99; Buy this book; 344pp) Detective Inspector Tom Thorne is in superb form attempting to solve a series of killings by a murderer who deliberately leaves a cut-off piece of an X-ray with his victims. The first body is that of a woman whose mother was the victim of a serial killer, Raymond Garvey, 15 years before. Thorne’s investigations run parallel with his developing emotional life, made complicated by his girlfriend’s miscarriage. He’s becoming more interesting and rounded with every book. Mark Billington’s previous novel showed how good he was without Thorne; Blood Line demonstrates his brilliance with him.

Also hunting down a serial killer, with equal skill, are Val McDermid’s oddball criminal profiler Tony Hill and DCI Carol Jordan, making their sixth appearance together. In Fever of the Bone (Little, Brown, £18.99; Buy this book; 432pp) their quarry kills teenagers of both sexes (unusual), hacking away their private parts yet not interfering with them sexually (also unusual); the victims appear to have nothing in common, except an internet social contact site. The police inquiries are hindered by a new chief constable’s wish to save money by not employing the freelance Hill, leaving the profiling to an incompetent young copper. Hill and Jordan — still not bedfellows — make a terrific couple. Their search develops as a brilliant succession of shocks and surprises. McDermid has never written better, which is saying something.

N. J. Cooper is the barely disguised nom de plume of Natasha Cooper, author of the splendid Trish Maguire series. No Escape (Pocket Books, £6.99; Buy this book; 325pp) is a departure in mood, setting and issues, but successfully achieved. The heroine is the psychologist Karen Taylor, conducting research into criminals with dangerous severe personality disorder. She interviews Spike Falconer — convicted of shooting dead a family of four — in Parkhurst, Isle of Wight. No reason or motive was ever discovered, and Falconer continues to claim his innocence. As she probes his mind, events occur that place her in danger and threaten her already vulnerable emotional stability. Slowly she begins to assemble the psychological and forensic clues, helped by her neurosurgeon lover and, more reluctantly, a local cop with his own problems. No Escape is an altogether satisfying novel, with the bonus of showing the Isle of Wight in new light — or, rather, darkness.

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