THE jacket cover is repulsive. It resembles the pop psychedelia used to sell Monkees' mysticism to 14-year-olds. If you bothered to decode the words "Incredible String Band," you still wouldn't buy--for fear of getting the New Christy Minstrels. The 5000 Spirits or the Layers of the Onion (Elektra Records) has been non-popular for months ("It sells about the level of Tim Buckley," reports a record store clerk); but it's of the same inventive class as John Wesley Harding and Sgt. Pepper.

The Incredible String Band is Robin Williamson and Mike Heron. They can play almost anything on almost almost anything--ranging from oriental to semi-calypso to blues, rock, classical guitar things and children's songs, blending all into their unclassifiable style using guitar, bowed gimbri, sitar, mandolin, flute, harmonica and an exotic percussion arsenal.

The ISB seems to have invented its own musical rules, and approaches each of its creations with the clarity and wonder of the voice in Williamson's "No Sleep Blues": "I mixed stones and water just to see what it would do; and the water it got stony and the stones got watery too."

SPARKED by brilliant musical performances and their sophisticated simplicity and variety of arrangement, the songs evoke, at different moments, Dylan's lyricism and the Lennon-McCartney precocity. Singing goodbye to "First Girl I Loved" (now 'a grown-up female stranger"), Williamson uses one of his awkwardly sensitive metaphors; "But in the white hills and behind many a long water, you have gathered flowers; and they do not smell for me." Heron, in "Painting Box," imagines himself out of a dark world: "My Friday evening's footsteps plodding dully through this black town are far far away now from the world that I'm in. My eyes are listening to some sounds that I think just might be springtime. With daffodils between my toes, I'm laughing at their whim."

Anomalous, intelligent, exuberant; perhaps important: More string than band, less string than incredible, incredible just the same.