Jack Reacher is a good read whenever you're not really in the mood for anything specific. Its simple and formulaic. Like chewing gum for the brain.
In the mood for a really sad love story?
The Fault In Our Stars. Its about teen cancer patients.
Modern Sci Fi about oppression and revolution?
Red Rising.
Young adult fantasy?
The Golden Compass. In it, everyones (soul?) is manifested outside the body as a kind of spirit animal. A very poignant story. Once I finished the trilogy, there was suddenly a gaping whole in my life