Reddit reviews How to Listen to and Understand Opera

Robert Greenberg's How to Listen to and Understand Opera is a good place to start. If you aren't already a subscriber, you can sign up for a free trial with Audible and listen to the audio book for free.

You could also watch the lectures for free via a free trial of Great Courses Plus.

1: Sign up to Audible - First book is free so you can / could cancel subscription after that

2: Download Professor Robert Grenberg's [How to Listen to and Understand Opera] (http://www.amazon.com/How-Listen-Understand-Opera/dp/B00DTO6IDC)

3: Enjoy over 24 hours worth of engaging instruction giving a fully comprehensive non-technical nuts and bolts introduction, history and thorough grounding in opera including biographies on key composers, vocal techniques, and all the various types of opera.

Hope this helps - it absolutely worked for me.