Cottingham's The Rationalists
Cottingham is an excellent interpreter of the Rationalists, especially Descartes who is his primary area of expertise. A lot of people unsympathetic to the Rationalists, straw man their position in books like these, or fail to understand, vital concepts like the concept of an idea. While I would not go so far as to say that I agree with everything that Cottingham writes here, it is the best general introduction to rationalist thought that I am personally aware of.
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 - John Cottingham)