The Grove Book of Operas, by Stanley Sadie and Laura Macy.
Serious, thorough, yet concise.

Opera (Eyewitness Companions), by Alan Riding and Leslie Dunton-Downer.
Quick, informative explanations and beautiful pictures.

Opera 101:  A Complete Guide to Learning and Loving Opera, by Fred Plotkin and Placido Domingo.
A friendly, exuberant book!

Opera: a Crash Course, by Stephen Pettit
A fun, quick guide to opera with many illustrations, recommended by Richard Beams.

A Short History of Opera, by Donald Jay Grout and Hermine Wiegel Williams
Respected classroom text, revised four times since original publication in 1958.

Who’s Who in Opera:  A Guide to Opera Characters, by Joyce Bourne.
Ever wonder how many opera characters were named “Leonora”?  Or want to learn the synopsis of, say, The Elixir of Love from one character’s perspective?  A highly entertaining directory.