For me it was Zen, which I heard is in some ways quite similar to Stoicism. A good start is Alan Watts' "The Way of Zen":

https://www.amazon.com/Way-Zen-Alan-W-Watts/dp/0375705104

The Way of Zen by Alan Watts is really good. I learned a lot and it helped me view the world differently.

http://www.amazon.com/The-Way-Zen-Alan-Watts/dp/0375705104

Alan Watts' The Way of Zen[1] is a great, rather iconoclastic, introduction to Zen and one if his best works. Another wonderful read is D.T. Suzuki's Zen and Japanese Culture[2]. And R.H. Blyth's Zen in English Literature and Oriental Classics is a jewell of a book.[3]

[1] http://www.amazon.com/The-Way-Zen-Alan-Watts/dp/0375705104# [2] http://www.amazon.com/Zen-Japanese-Culture-Bollingen-Series/... [3] http://www.amazon.com/Zen-English-Literature-Oriental-Classi...