If you’ve just started a trilogy of books, you’ll be reading for a while. A trilogy is something that has three parts, so you'll have three books to read.
You can see the tri-, meaning "three," in trilogy. The word originates from the Greek word trilogia, meaning "series of three related tragedies performed at Athens at the festival of Dionysus." Today, you can use trilogy to describe three of anything, such as a trilogy of movies that has the same characters, settings, and themes.