Personally I recommend Martin Heidegger's "Being and Time" as well as Brian Greene's "Fabric of the Cosmos." Hedeigger's masterpiece cover's his exploration of the question of the meaning of "being." This might sound vague, but essentially he's asking questions about the meaning (and significance) of existence itself. Greene's work is a much more straightforward discussion of theoretical physics and the theories of space-time, general and special relativity as well as other related topics. At times things are simplified a bit too much, but overall the conceptualizations of a very hard to grasp topic are helpful