My interpretation of Durkheim's idea of "our social environment is filled with things only real in our minds", is that society is built and structured by ideologies/institutions that are not necessarily "real"...for example, a business is considered a separate entity other than the people or person who own the company/work it, etc. The company is not an actual living being, but we have created meaning to it by calling and treating it as a being.
The earlier examples of passport and money are wonderful as well... WE as humans add meaning to these ideas and objects.
Society is only constructed from our projected ideas / meanings

John Gray's "Straw Dogs" is a good one
I read a lot of Simone de Beauvoir, Sartre and Camus...
"At the Existentialist Cafe" by Sarah Bakewell is a good overview of existentialism and how the main thinkers are all connected... pretty much like everyone else said, if a book interests you, keep reading. However, I do find that reading some main philosophers like Heidegger, Husserl, Beauvoir, Sartre, Hume, Hegel, etc. is a good way to not get super lost in some material, as they do show up often
Oh one more good one, "Evil in Modern Thought" by Susan Neiman