Video below discusses my favorite operatic reference books

Where to buy books online:

My favorite site for buying used books online is Biblio.com.
Biblio supports the independent booksellers on its site, and
does wonderful things worldwide to encourage literacy.
It's an easy shopping experience, too. No hassles.

Biblio.com

Here also is the link to Amazon,
but it is the "smile" Amazon site, which allows you to
choose a charity to benefit from your purchase if you want to.

Smile Amazon

And here is a site to help you
find an independent bookstore near you:

IndieBound

Here is a list of my favorite
general purpose reference
books for opera

New Kobbé's Opera Book
Earl of Harewood and Anthony Peattie Editors
ISBN 978-0091814103
 
Great Operas Volumes I, II & III
By Ernest Newman

Wagner Operas (Also published as Wagner Nights)
By Ernest Newman
 
A Passion for Opera: Learning to Love It
By Peter Fox Smith
ISBN 978-1570762802
 
The Operagoers Guide:One Hundred Stories and Commentaries
By M. Owen Lee
ISBN 978-1574670653
 
Also mentioned in the video and recommended:
Stand Up Straight and Sing!
By Jessye Norman
Paperback ISBN 978-0544484054