Spinoza's "Ethics". It was one of the first efforts to apply mathematical logic to philosophy (Wittgenstein borrowed a lot from it), and to provide an axiomatic model of emotion. Very roughly, it took a two pronged approach to illustrate there is no objective logical foundation for differentiating between "me" and "not me": firstly, by showing that, given determinism, everyone/thing is part of a single unalterable process, and secondly, by showing that there is no single objective standard for drawing the line between a thing and the things around it. (He doesn't deal with nondeterminism, but most of the conclusions could be mapped directly to a system containing a mix of nondeterminism and determinism, as adding nondeterminism cannot "increase" moral responsibility or "self-causation").
In that sense it gave a (comparatively) rigorous argument for the nebulous eastern concept of "oneness". Using this foundation he then makes a logical argument (given his axiomatisation of emotion) for why we should be happy.
It's also the book that made me realise that, even if the jealous Abrahamic God exists, He is not moral. Because due to determinism every action can be traced back to the initial cause, so it doesn't make sense for Him who created the universe to punish or reward people for actions that were ultimately predictable consequences of the universe's creation, which if omniscient he should have foreseen. (And if some things are nondeterministic, this still applies, because somebody cannot gain moral responsibility through decreased determinism).