Interesting and wild ideas in this book. I read most of it, but I had to skip some of Spinoza because I ran out of time and also his writing is not very exciting. Interesting nonetheless. All three of these guys use arguments to prove the existence of God. "Contigo ero sum" or "I think therefore I am", Descartes famous line, that which cannot be doubted is that I exist when I have this thought that I exist. Even if there is some evil demon with complete control of my body and mind, something still necessary exists when it thinks it exists.
Spinoza's idea that God is everything is kind of neat. Also that there is no free will is pretty wild, not even God has free will So he created the universe out of necessity, not because he wanted to. It could not have been otherwise.
Leibniz is a determinist like Spinoza, but he has this "incline without necessitating" thing that makes people morally responsible for thier actions even though they could not have done otherwise. When creating the world, God chose the best possible word. When God was choosing which world to create out of all the possible worlds he used his wisdom to know the good (he doesn't create the good, it exists separately from God), his goodness causes him to choose it, and his power enables him to produce the best possible world.