One who takes in and interprets what was written. The reader completes the writing, and no two complete it the same way.
Old English rædere (one who interprets, an adviser), from rædan (to read, to advise) + -ere (agent suffix); a reader was named, from the start, as one who makes sense of something rather than one who merely receives it.
One who takes in and interprets what was written. The root's own adviser sense is fitting: the reader completes the writing, and no two readers complete it in quite the same way.