The names of John Constable and Joseph Mallord William Turner have been linked so closely for so long that it comes as a surprise to learn, about a third of the way through Nicola Moorby’s engaging joint study of Britain’s two greatest painters, that they didn’t meet until 1813, when the two men had been in London pursuing artistic glory with varying levels of success for a decade and a quarter-century, respectively. Given their vastly different temperaments, personalities, upbringings, and aesthetic sensibilities, perhaps the bigger surprise is that they should have met at all. As near as they were in time and space, they remained, thanks to their contrasting and conflicting visions, worlds apart. Yet different as they were, they were bound by a mutual conviction in the dignity and validity of landscape not only as a legitimate genre of painting, but as a distinct and unique art form in its own right.