Here's a lecture (sound recording) by my neighbor Nancy Kendrick on Descartes, Spinoza, Leibniz. (Sound values not so great : adjust volume down...) Kendrick is chair of philosophy dept. at Wheaton College.
There was a recent book on the personal/professional relationship between Spinoza & Leibniz - will try to find reference (OK, here it is : The Courtier and the Heretic, by Matthew Stewart). A revisionist-historicist's view(?). Want to find out.
The description of Descartes' proof of God's existence (God = "that which must be", etc.) - reminds me of the passage in the Gospel of John where Jesus says "I am the Way, the Truth and the Life." (That which must be = Truth.)
When I was a teenager I thought the fundamental problem in the universe was this : what is the basis of reality - consciousness or matter? I came down on the side of consciousness. I have a clear memory of walking away from camp one evening, on a remote lake in Canada (1967 or so) - looking out at the water, thinking about this.