Leonardo's "Last Supper," perhaps the world's most famous painting, no longer really exists: restorers are responsible for as much as 80 percent of the painting we see today at Santa Maria delle Grazie in Milan. Its degeneration began quickly: Because Da Vinci did not paint it in fresco (in which pigment is applied to wet plaster, permanently bonding it to the surface) but rather in oil, the work never properly adhered to the wall in the first place. A "perfect storm of adverse conditions" -- damp, mold, soot, smoke and steam -- meant that about 80 years after it was completed, the painting was already reported to be "in total ruin."

And then things really got bad. The friars cut a door into the wall, amputating Christ's feet and loosening the pigment still further. One of Napoleon's soldiers used the site as a stable. Clumsy restorations -- in some cases verging on the criminal (one restorer turned Thomas' hand into a loaf of bread; another tried to remove the whole work from the wall and ended up gluing back the fragments) -- completed the decline. A (perhaps) final high-tech conservation/restoration effort was unveiled in 1999. This sensitive and scholarly attempt to stabilize the painting and return it to its original condition, Ross King explains, can be likened to the ship of Theseus, "the vessel carefully preserved by the Athenians, who eventually replaced every one of its rotting timbers and thereby caused philosophical dispute about whether or not it was still the same ship."

Existential discussions aside, "The Last Supper" is always lauded as a masterpiece, a symbol of the greatest art human beings can achieve. But why? And how did it happen that a 43-year-old procrastinating genius who had started dozens of projects but had finished almost none -- and who really wanted to create great war machines, not frescoes -- came to paint this icon of Western art? King's vibrant new book, "Leonardo and the Last Supper," weaves together history, biography and art criticism to answer these questions and many more.

King does an admirable job of delineating the tangled politics of this period when enmities and alliances were in continual flux and the wheel of fortune rotated at great speed. But the book is most compelling as it unpacks the painting itself: its compositional genius, its doctrinal stance (seems Leonardo was much less heretical than is often asserted), its psychological realism. A particularly fascinating chapter is devoted to food and drink (of course bread and wine feature prominently, but who knew the entree was a platter of eels with oranges?); another interprets the "language of hands."

As to the identity of the Apostles, well, sorry, Dan Brown, but that rather feminine looking apostle in Leonardo's "Last Supper" is almost certainly not Mary Magdalene; Leonardo's gender-bending figures were of a much deeper -- and probably more subversive -- order than the simple substitution of female for male. But as King acknowledges, Leonardo's masterpiece "has become its own story," open to endless interpretations and reinventions. "Leonardo and the Last Supper" is an engrossing account of this great story.

Patricia Hagen teaches English at the College of St. Scholastica in Duluth.