Didn't downvote you, but while I've heard similar recommendations before, that's not a reasonable response to existential depression. On the one hand, you're c

Didn't downvote you, but while I've heard similar recommendations before, that's not a reasonable response to existential depression. On the one hand, you're correct, one's outlook on the world does impact your mood. Humans are emotional and easily confused, and shifting your viewpoint can change your mood in powerful ways. Perhaps those books present a position from which to view the world that's useful in lifting one's mood, which shifts your attention away from things which are depressing and towards things which are uplifting. On the other, if there are genuine facts which are important and which cause me to be depressed, it is important to me in my core values to believe those things to be true, neither blinding myself to their existence nor incorrectly believing them to be false. Changing a viewpoint will not cause those inconvenient facts to become false: They exist, they cause depression, I am aware of them, hence I am depressed. I am willing to add facts to my worldview which are uplifting and counterbalance the depressing ones. I am willing to adjust priorities and weigh the importance of the depressing ones less and the uplifting ones more, in proportion to the degree that they are or are not important. I might be willing to cheat a small amount, and allow myself to lean my viewpoint slightly to the happy side of the correct position if that position is more productive, but there's a fairly low limit to the self-deception I can tolerate, even if it causes me to be depressed.