A book by Martin
Heidegger, famous 20th century
German philosopher.
This is a really big book which I don't pretend to understand completely. But it has some fascinating concepts worth mulling over. He was gripped by the question of what we mean by "being," and spends most of the book talking about Dasein or "being there."
Because the book deals so much with semantic nuance, it is much better to read it in the original German.