The Invention of Dr Cake, by Andrew Motion

Jonathan Bate is moved by the beauty and truth of Keats reimagined

Biographers are notorious for falling in love with their subject. Novels such as Possession and The Hours turn on the obsessive nature of literary research. Andrew Motion's new novella enters into this arena, which could perhaps be described as "parabiography." It proves to be the cleverest and most interesting thing he has written.

The book is the logical next step in a sequence of prose works that began with a wholly orthodox biography of John Keats and proceeded to Wainewright the Poisoner, which was half biography and half feigned autobiography of one of the most slippery and colourful characters in the Keats circle. Motion has now turned the wheel from half fiction to full: he invents another "minor Romantic", one William Tabor – physician and Keatsian poetaster – who himself invents the mysterious Dr Cake.

Cake is a worthy doctor living in rural obscurity in the early 19th century. He has a highly literary sensibility and, in the course of conversations with Tabor, comes to sound more and more like John Keats, speaking of Shakespeare as his "Presider" and of human life as a "large mansion of many apartments". His life is curiously parallel to that of Keats: born in 1795, studied medicine at Guy's Hospital under the auspices of the great surgeon Astley Cooper, travelled to Italy.

It is hardly giving away the plot to reveal that Tabor comes to believe that Cake really is Keats. Suppose the young poet recovered from his consumption and walked alive from that suffocating room under the Spanish Steps in Rome. Suppose that he felt he did not have the strength to go on being crucified by the critics or that his muse dried up with the loss of his beloved Fanny Brawne, or that he foresaw a decline into poetic mediocrity, or that his near-death experience made him decide to return to his medical vocation and save other lives instead of fannying around with effete poems on the subject of swooning bosoms and flitting birds. Could he then have returned to England and assumed a new identity?

The conceit allows Motion to explore several related themes: the idea that the Romantic poet's best career move is to die young (contrast the glamour of Shelley and Byron's early exits with the long decline of Coleridge and Wordsworth), the notion that writers survive through their "afterlife" in the work of their imitators (and biographers), the question of whether writers actually achieve any good in the world, or whether they would do better to take up a practical calling such as medicine.

Motion is a professor of English literature: many of his peers have written theoretical treatises on these questions. He is also a poet laureate: many of his peers have written sub-Keatsian verse like that of Tabor. By turning to the form of the novel, he has found a most elegant third way. The Invention of Dr Cake works as both a mystery story and a meditation on the nature of poetic identity.

The writing is atmospheric, with the period detail effectively done. The outline of the career and publications of "William Tabor" is so plausible that one almost finds oneself turning to the British Library Catalogue just to make absolutely sure he didn't really exist. But a few deliberate mistakes are inserted so as to puncture the illusion: thus "poor John Clare" is numbered among the poets who died young, whereas his tragedy was that he lived to a great age in a lunatic asylum. Wordsworth's infamous sinecure as Distributor of Stamps for the county of Westmorland is described as work for the Post Office – whereas it was really an Inland Revenue post, involving the collection of stamp duty.

Jonathan Bate's biography of John Clare will be published in the autumn.