Laura Cumming’s The Vanishing Man: In Pursuit of Velázquez is a tale of two devotees of the great Spanish court painter, one a 19th-century bookseller from Reading called John Snare, the other the author herself. What unites this odd couple across more than a century is their shared passion for the artist. When Cumming contrasts Snare’s fervour for Velázquez with the orderly deliberations of art historians, she is also speaking of herself. Snare was, she says, ‘looking at a single work of art with the power to affect the viewer as a person or a poem might … He is an evangelist for his Velázquez: he wants … the world to see and love it as he does.’ The only difference is that Snare was a marginal figure obsessed with a single painting, a putative Velázquez portrait of the young Prince Charles completed in 1623 during the future king’s fruitless visit to Madrid in pursuit of a Spanish marriage, while Cumming, as The Observer’s art critic and author of a highly regarded study, A Face to the World: On Self-Portraits, has much more authority to speak about Velázquez’s work as a whole.

Cumming deftly moves back and forth between the sad story of Snare’s efforts to authenticate ‘his’ Velázquez and her own enthusiastic and vivid account of many of the artist’s greatest works, but the two parts differ in tone. Cumming’s telling of Snare’s tale reads like a skilfully plotted detective story