Apart from philosophy mentioned by the other poster, you can also read into psychology. Some branches in psychology deal with the question of how to live a fulfilled life, eg. positive psychology [1] and humanistic psychology [2]. Also Buddhism has similar goal and teachings interleave with these psychology topics [3].

To start, you can pick a Psychology 101 textbook (I like Psychology and Life [4]) and then read the people and bibliography mentioned in these Wikipedia articles.

[1] https://en.wikipedia.org/wiki/Positive_psychology

[2] https://en.wikipedia.org/wiki/Humanistic_psychology

[3] https://en.wikipedia.org/wiki/Buddhism_and_psychology

[4] https://www.amazon.com/Psychology-Life-Richard-Gerrig/dp/020...