Roger Fiske's SCORE READING series is meant for classical music listeners who have some very rudimentary musical training (time signatures, scales, and the basics of harmony) and want to learn how to read an orchestral score while listening. In the first volume, "Orchestration" Fiske teaches by means of the slow movements from several standard repertoire pieces. You'll have to provide your own recordings, but these pieces are readily available from libraries if not already in your home collection.
Fiske starts with the simplest of concepts: following a melody. Anyone can learn to do this in seconds. He then moves on to following two staves, and here his advice to watch the bass until the "tune" is clarified will provide enlightening to beginners. Note that with its emphasis on "tunes" held by one or two instrumental lines which the reader can focus on, Fiske's series is certainly of limited use in learning to follow 20th century scores, though Oxford University Press eventually added a fifth volume for that purpose written by Malcolm Barry and Roger Parker.
Most useful for me were the remarks on the writing for individual instrument families in the orchestra. Brass instruments are notated at a different pitch than they actually sound, and Fiske will tell you why that is and how to deal with it.