I recently posted a review of The Achilles of Rationalist Arguments by Ben Mijuskovic (here). This book deals with the history -- primarily in the early Modern era -- of the idea that "The essential nature of the soul consists in its power of thinking; thought, being immaterial, is unextended, i.e., simple (having no parts); and what is simple is (a) indestructible; (b) a unity; and (c) an identity."
I have recently been pointed to this summary of Mijuskovic's writing on the Simplicity Argument, including another book, Contingent Immaterialism: Meaning, Freedom, Time and Mind, which looks pretty darn interesting. It also looks at recent developments in this argument, including a collection of essays entitled The Achilles of Rationalist Psychology. There are many more references at the link. It looks like my reading list just doubled.
(cross-posted at Quodlibeta)