Composer biographies often include fairly extensive information about the immediate context surrounding the creating of an opera - where was the composer living, who and what were the influencers in the composer's life at that time, what were the major personal, social, cultural, political events affecting the composer, etc.
Pro Tip - since time is a luxury at this point, only read the portions of the biography that relate specifically to your opera. Discover your target pages by going straight to the index at the back of the book and looking up your opera.