Heinlein used to write about "spools" and "projectors," back in the late '50s and early '60s, but I think he was thinking of a variation of microfilm. (Maybe not, though -- the hero of one story carries a secret message magnetically encoded onto a microscopic piece of wire.) The incredibly dense book/library in Frank Herbert's
Dune also used optical analog technology, rather than electronics. James Schmitz had a portable law library (in the Telzey stories) with an unclear storage technology, though again microscopic spools of some kind were involved. I'm trying to remember the name of the author and book that had the ereader device run a test as you started reading to determine your maximum intake speed, and it would adjust to compensate. I think I read it in the early '90s. It's probably in a box in the basement.
