The mood of a piece is how you feel as you read it. Authors create mood through the use of tone, description, subject matter, characters and theme. Some pieces of literature are clearly designed to make readers feel good, while others are written to make readers feel pensive, sad, thoughtful or frightened. The mood of a piece isn't necessarily what you are thinking about when you're done reading, but rather a more generalized sense of how you feel.
Moods are powerful tools for a writer to use when taking a character through the plot of a book. For instance, a story that has a clearly happy or positive mood at the beginning may turn suspenseful or melancholy as the story progresses. As characters endure hardship, a writer will usually make a conscious effort to create a darker mood to draw an atmosphere for the reader that emphasizes the experiences of the characters.
The setting of any particular scene in a book plays a large part in what mood is established. For instance, the mood a reader feels at a hospital when a baby is born is much different from the mood he will be likely to feel when he reads a scene set in a cemetery. Of course, this same tool can be used to create contrast, such as when a humorous scene is set in a gloomy or oppressive setting.
When writing about the mood of a piece of literature, it is important to be able to do more than simply indicate what you feel the mood of a piece to be. Instead, point to the elements of the story that create the mood. Look for specific word choice, plot points or descriptions that help create the mood. Keep in mind that mood is also somewhat subjective, making identifying the key mood-influencing elements important for also deducing what other moods a person might feel in the piece.