All of it. Without knowing who this fella is, what his interests are, how much philosophy he's read in the past, here are some suggestions without commentary, justification or interpretation.
1) The Republic, Plato --- also the Symposium and the Sophist.
2) Nietzsche, anything really but esp: Genealogy of Morals, Thus Spoke Zarathustra, Twilight of the Idols, and the Anti-Christ.
3) Soren Kierkegaard: Either/Or.
4)Lucretius: On the Nature of Things
5)Derrida, Jacques: Memoirs of the Bling.
6) Foucault, Michel: Birth of the Clinic, the History of Sexuailty
7) Spinoza: Ethics.
Too many others, I'm sure.
Because if you try it the other way around, your shoulders will really hurt.
I could fall in love with any man I wanted to, the problem would be whether he'd fall back in love with me (probably not).
You are not mistaken. He doesn't see it as particularly bad (well not the Spinozist conception --- I gather the Einsteinian might be more problematic, but I don't know enough about it). That is to say that I'm committed to the idea of the unity of substance, but I tend to think that a) the necessary duality of knowable attributes and b) a reading of e2p7 (the order of things and knowing is the same or something like that) that sounds like attribute parallelism are both bad, and end up reproducing the problems of theism within a supposedly atheistic regime --- think of the philosophes or of the neo-Darwinians.
On the other hand, Nietzsche, Klossowksi et. al. have done fantastic work for atheism through embracing key aspects of Spinozism.
Finally, I do believe in the Greek gods. F'real. And this does relate to what I mean by rejecting a duality of attributes, but why takes too long to explain.