Not sure what counts as ambient and what doesn't, but here are some suggestions for "music that is there, but not getting in the way of reading, talking etc. ..."
Steve Roach, particularly his CD Structures from Silence (once you hear it, you'll know how perfect the name is)
Music of Alan Hovhaness, especially the selections featuring harp
Bill Laswell, Ambient Compendium
Just about any good solo acoustic guitar recording. Several of the guitar and lute recordings on MA Recordings are favorites of mine for this type of listening.