By making the central character [of Birthstone] a split personality [D. M. Thomas] attaches his book to a long line of doppelgänger fictions, and thereby applies for a certificate of profound intent. Conrad's The Secret Sharer, Wilde's The Picture of Dorian Gray and Stevenson's Dr Jekyll and Mr Hyde are obvious ancestors, and behind them—stretching right across Europe—stands a host of others. But while Thomas implicitly refers to them, he seldom allows himself to make the most of their example. Where they are expansively fanciful he is inhibited by a fear of seeming absurd; where they are chillingly analytic he is rather ponderously explanatory; and where they successfully blend fantastic with realistic worlds he wanders uneasily between the two.

Saying so makes Birthstone's uncertainty sound a debilitating weakness. Given the mental state of its heroine and narrator, Jo, it is in fact the novel's...