DCI Hannah Scarlett's latest cold case concerns Bethany Friend, a young woman who apparently committed suicide half a dozen years previously by drowning in the Serpent Pool, a local beauty spot by a ruined tower. Bethany's frail old mother has never believed the girl killed herself, and because the death had some unusual elements (the girl's hands were tied and she was known to be scared of water), Hannah and her new, not very pleasant sergeant, Greg Wharf, believe there is cause for doubt. They re-open the case, attempting to trace and re-interview all the girl's known contacts.

In the meantime, Hannah continues her troubled relationship with her partner Marc, owner of a local bookstore and antiquarian book business. She's physically attracted to Marc but has stopped liking him; and has fallen in love with local historian and TV celebrity Daniel Kind. Daniel has recently returned from a sabbatical in the United States and is newly single. He's accepted a commission from charity campaigner Arno Denstone to write and present a paper at a festival Denstone is organizing to celebrate the life and works of Thomas De Quincy, the poet and author fascinated by murder as well as being a famous opium addict.

THE SERPENT POOL opens with a present-day murder, that of estate agent George Saffell, one of Marc's best customers. Not long after, Marc and Hannah attend a party at the mansion of a solicitor and another of Marc's rich customers, Stuart Wagg, and witness an angry scene between Saffell's widow and Denstone. Hannah discovers that Wagg's latest female companion is Louise Kind, Daniel's sister, and thereby discovers that Daniel has returned from the USA to his Lakeland cottage.

The following morning, Daniel is woken early by a distraught Louise. She and Wagg have had a terrible row, resulting in Wagg ending their relationship and Louise attacking him with a pair of scissors and running off. Terrified that she has caused a serious injury, she persuades Daniel to go back to the house with her to check that Wagg is all right. They can't find him, however, and the house is locked up. Worried about his sister, Daniel seeks out Hannah to ask her help.

As Hannah continues her investigation into Bethany's disappearance and searches for Wagg, she gradually becomes certain that the old and new cases are linked. She and her colleague DCI Fern Larter, who is getting nowhere with her investigation into Saffell's death, decide to combine their resources, and soon the clues begin to make themselves known, escalating to a multilayered, tense and compelling climax.

THE SERPENT POOL is the fourth in the author's excellent Lake District series, and I think the best yet. Most of the absorbing first half of the novel is taken up with Hannah's and Daniel's separate preoccupations and emotions, as they go about their daily lives in strong, but unspoken, awareness of each other. The novel is infused with many small but telling observations about society, literature, the book business and police work, at the same time building up a sense of tension as we realize that an old mystery is continuing to have dramatic impacts. In addition, Hannah is constantly uneasy about Marc - and because her own feelings for him are changing she does not know how far to trust him, or what he might have done in the past. When another murder happens about half way through the book, the mood shifts from introspective to action-based. The author has woven a clever and very tangled web to keep his readers fully distracted before the final solution is revealed.

I highly recommend the Lake District series, probably best read in order though it is not essential. As well as being an excellent set of page-turners replete with authentic local colour, the books have the added benefit of being written with intelligence by a talented author who is clearly very knowledgeable and cultured, with a mature, gently ironic view of the world. I congratulate Martin Edwards on this superb contribution to what I think is one of the very best crime-fiction series being written today.