The book is now in store at the Juilliard Store in New York, which stocks music for the Juilliard School:

https://juilliardstore.com/products/opera-the-autobiography-of-the-western-world

And the book is on sale at the Royal Opera House Covent Garden, where it became the bestselling book on opera:

https://shop.roh.org.uk/collections/opera-books/products/opera-the-autobiography-of-the-western-world-book