I’m a big fan of the audio courses from The Teaching Company. I usually go to Amazon.com to buy used versions, though I’ve gotten some courses at the website when they’re offered on sale (which happens often).
Not everybody likes him (he has a thick New Jersey accent, and a corny sense of humor). As the courses get more in depth, though, he’s less jokey.
Anyway. The current course I’m working through is “The String Quartets of Beethoven”. Today was a lecture on opus 127. Apparently this music was a bit ahead of its time Greenberg quoted one of the contemporary reviews.