Sarah Caudwell's last mystery is a delightful brainteaser

Readers who like well-written mysteries that are also brainteasers will find Sarah Caudwell's "The Sibyl in Her Grave" (Delacorte, $23.95) at the top of that genre.

This fourth and -- alas -- final mystery by Caudwell, who died in January, might encourage some to read or even re-read the previous three.

Two worlds intersect, even tangle, in this book, which is recounted mainly through calm letters.

In Parsons Haver, a village in Sussex, Regina Sheldon writes letters to her niece Julia Larwood, a lawyer in London. They're read also by some young lawyer friends and by professor Hilary Tamar, a recurring character whose gender Caudwell never has specified.

The letters tell about a fortuneteller, also a blackmailer, who has moved to Parsons Haver with her niece. The fortuneteller dies, after having been visited periodically by a man in a large black Mercedes. However, she had passed on amazingly successful investment advice to a small investors' group to which Regina belonged.

Meanwhile, Selena, another London lawyer, has been consulted by a bank chairman who wants to resign and name his successor. The likely candidates are two men, but the chairman suspects one of them of insider trading. He hopes Selena can figure out which one it is.

The lawyers and Tamar meet and speculate that one of those candidates provided the insider trading information in Parsons Haver, was being blackmailed and murdered the fortuneteller. But which candidate?

They read more of Regina's letters and formulate several theories. The letters are genteel and fun to read, with an overlay of humor. One letter, from a lawyer spending Christmas on the Riviera, ends with this cliffhanger: "On my way back (from the postbox) I shall knock discreetly at her door. If there is no answer and the cries are still continuing, I shall climb across onto her balcony, which adjoins this one, and find out what is going on."

Situations intertwine and become complicated. A carpenter engaged to build bookcases in the law offices falls in love with a Parsons Haver resident. A minister dies in Parsons Haver, and so does the fortuneteller's niece.

Are these murders? Some are. Ideas and possible solutions tease the brains of the little group in London. And of the reader.