brian eno had broken his ankle, and was brought back in plaster from the hospital by a couple of friends who got him comfortable on the couch before going off to do the shopping. Before they left, they opened the windows of the flat, and put something on the record player. Eno sits back once they have gone, his foot elevated, and notices that some bach was playing, but one of the speakers wasn't working, and the music was on very quietly. The sounds of the music blended with the sounds of the city coming through the windows, and ambient music was born......music that is beautiful if you listen, but it sits in, enhances the environment when played quietly so that it can be ignored.
The second album in the ambient series Eno made with a little known american pianist called Harold Budd, and it remains a piece of beauty, the Plateau of Mirror. They made a second piece later called The Pearl. This is the only music that I have never tired of, partly of course because Eno designed them that way. On Land, as someone mentioned above is extraordinary, music made from the sounds of life, or the other way round. Eno's ambient work is music of the highest order.
The other little known ambient music is robert fripp's frippertonics work, ( Let the Power Fall, A Blessing of Tears ) which is easily the most hypnotic music made, but eerily beautiful, all live looped delayed guitar. Do not drive to this music. Fripp and Eno made a couple of pieces together like 'No Pussyfooting' and 'Evening Star', which are as harsh and dark as the Plateau of Mirror is warm and light.
Last, Gavin Bryars, 'the Sinking of the Titanic' takes ambient into a whole other place, weird and wonderful.