Britain's landscape; Britain, by David Gentleman. New York: Dodd, Mead & Co. 224 pp. $27.

David Gentleman's ''Britain'' is sad, yet instructive, an elegy to the ruin of that country's landscape. ''It is not merely that we build too much or too fast but that we do it shoddily and to cramped standards, replacing what was flawed but reparable with things that show their flaws even quicker and are harder to get rid of,'' he writes.

This is why his use of contrasts is so telling in this traditionally structured sketch-cum-guide-book to Great Britain. The artist-illustrator juxtaposes for example: Greenham Common (the US missile base) and Royal Windsor Castle; Cowley's British Leland factory buildings and Oxford's gracious colleges; Shropshire's Iron Bridge (with its ''delicate decoration, giving one something to enjoy as one crosses'') and Motorway 6 near West Bromwich (ugly and graceless); ancient Hampton Court and the 1930s Hoover Building.

But there are strange affinities as well between the landmarks in these pages - for example, in England the Devil's Chimney on Leckhampton Hill above Cheltenham and the Victorian Tweed Mill at Chipping Norton; or in Wales, Llanchony Priory in Gwent and the viaduct and aqueduct at Chirk, Clwyd.

Very effective, too, are Gentleman's depictions of the patterned valleys of Wales and the field near Hackpen Hill in Wiltshire sketched just before a storm, set off by black, threatening clouds. Gentleman's views of the front doors of houses in Dublin are an aesthetic delight, while his rendering of Donegall Square in strife-torn Belfast, with a heavily grilled police van and checkpoint, is ineffably sad.

This is an attractive yet disturbing book, which is apt to make American Anglophiles want to see Britain once more before it is carved up beyond recognition. One is left reflecting on the relationship between art and landscape. As Gentleman writes, it is the old master artists (Blake, Constable, and Turner) who ''enhanced our awareness of what surrounds us,'' showing ''us certain aspects of the landscape so compellingly that they might almost have invented them.''