Having made it to chapter three of Arthur Melzer's Philosophy Between the Lines, I find that the book has taken a surprising turn for the better. I say surprising, because after the weaker material in chapters one and two I did not expect to like chapter three as much as I do.
The problem I see with chapters one and two is that I think Melzer overstates his case. For his main argument, which I feel is found in chapter three, to go through, all that he needed to claim was that there are frequent and important instances of esotericism classical and Medeival thought. And that claim he can back. Instead, he claims that it "was a nearly universal practice among Western philosophers prior to the late modern era" (p. 69), a claim that is well beyond the evidence he presents. And trying to back it leads him to stretch the term "esoteric" beyond its useful limits.
And it is particularly unfortunate that Melzer leads with this weaker material, since chapter three is so strong. Its central claim concerns the difference between classical rationalism and Enlightenment rationalism: "this new enlightenment rationalism goes decisively beyond Plato and classical rationalism by claiming that theory can not only free itself from praxis but ultimately return to, rule, and rectify praxis" (p. 81).