Why not start with the #1 NY Times best seller for non-fiction for 2016: At the Existentialist Café: Freedom, Being, and Apricot Cocktails. https://www.amazon.com/Existentialist-Caf%C3%A9-Cocktails-Jean-Paul-Merleau-Ponty/dp/1590514882/.
I read it an loved it.
Another good place to start is Robert Solomon's lecture series: No Excuses No Excuses: Existentialism and the Meaning of Life http://www.audible.com/pd/Nonfiction/No-Excuses-Existentialism-and-the-Meaning-of-Life-Audiobook/B00DDY7SWS