Whenever I feel down, I remember one powerful book I've read: "White on Black" [1]. It is in Russian and was written by a person born with a cerebral palsy, who had survived in soviet orphanage. The very vivid depiction of a life of a person with severe disability just forces you to better appreciate things you are taking for granted: like a capability to walk on your own and have working hands. This is what matters, and all other problems are transitory and should be treated as such.
Unfortunately, I don't know if a translation to English exists.
In a similar vein, Man's Search for Meaning by Viktor Frankl. It's about an Austrian psychologist who experiences the holocaust firsthand. It's partly a description of the events that unfold, but also his thesis on his work called "logotherapy". A book about a person surviving the holocaust might strike you as really grim, but he brings a lot of humanity to the situation and it really makes you appreciate the life you have.
Unfortunately, I don't know if a translation to English exists.
[1]: https://ru.wikipedia.org/wiki/%D0%91%D0%B5%D0%BB%D0%BE%D0%B5...