I checked out Amazon and found this highly-rated book:
Opera 101: A Complete Guide to Learning and Loving Opera
I can't say I have any experience with the book myself, but it seems to do all the right things.
I also perused the "A-Z Opera" selection and it looks to be a pretty good collection, but I heartily encourage you to go beyond what it offers, especially since the collection ends, chronologically speaking, in 1926. (Perhaps the booklet has some suggestions for further listening?) Several post-1930 operas have been mentioned in the previous pages, and many of them are not only perfectly accessible, in spite of the reputation that 20th century music has (deservedly) earned, but also more-than-worthy "bearers of the operatic tradition." (One frequent "cocktail party conversation" among opera buffs is deciding what is the last
opera, chronologically speaking, that can certifiably be said to be part of "the performing canon.") The timeline that you mentioned offers several good places to start.