The first two Gormenghast books, by Mervyn Peak, feel weirdly appropriate for me. They have a villain whose effectiveness depends on his rationality, when all about lack same. The more potent advocate for rationality, however, is the compelling depiction of a world devoid of it, where the shackles of convention enslave the castle's populace.

I dunno, ymmv, but I implore you to give them a shot. They made a lasting impression on me, in a way that many books don't.