They are a series of 6 full length university lectures from Harvard from 1973. I owned them first on LP (long play vinyl albums), and they have always been available in book form.

http://www.amazon.com/Unanswered-Question-Harvard-Charles-Lectures/dp/0674920015/ref=sr_1_1

They are also available on DVD.

http://www.amazon.com/Unanswered-Question-Harvard-Leonard-Bernstein/dp/B00005TPL8/ref=sr_1_2

Youtube copy if you want to preview them first...

https://www.youtube.com/user/cagin/videos

Some parts get complicated, particularly if you not a musician, but they are very well worth it overall.

Bernstein's famous Young People's Concerts are also worth a watch!

http://www.amazon.com/Leonard-Bernstein-Peoples-Concerts-Philharmonic/dp/B0002S641O/ref=sr_1_1

http://www.amazon.com/Leonard-Bernsteins-Young-Peoples-Concerts/dp/B00E6DMZA0/ref=sr_1_3