Saint Leibowitz and the Wild Horse Woman is, in my view, a seriously flawed book with portions which are brilliant, and others which are nigh unreadable. However, it does tend to polarize people, so if I were you, I'd give it a try, as you may end up in the camp which supports the novel.
And Parson and I have discussed this elsewhere at length, but it is unlikely that Miller (at least at this point in his life) was intending any sort of attack on religion; nor is Benjamin (the Wandering Jew) what one could call "a racist inside joke"; this is a figure which has appeared throughout literature since the Middle Ages, both for literal and metaphoric purposes; and he is a very complex character (and symbol) here. To belittle either the character or his role with such a phrase is, in my view, to seriously miss the point. As has been said, a complex and subtle book of many layers, one well worth multiple readings and ponderings.
(Incidentally, I would demur somewhat about it having a "playful style", at least as a whole. Certainly, the first book ("Fiat Homo") has much of that, though there are some rather grim moments even there. The second ("Fiat Lux") begins to blend the humorous tone with a more sober one, as the theme, originally showing some hope of light, begins to darken (making the title of this section both ambiguous and ironic). The third ("Fiat Voluntas Tua"), though having its lighter moments, is predominantly dark and grim, yet offers its own kind of "light" or hope in the figure of Mrs. Grales' twin, a figure of purity and innocence which also -- again complexly -- stands for the disfigurement of Man due to the fall (physical deformity, though the twin is, as noted, purity itself; even, it is hinted, the Second Coming; and also the fallen yet oddly innocent character of Mrs. Grales).
It is interesting that, as an atheist, I am so fond of this novel because it treats both its themes and its characters with such respect and compassion; which is something I found rather lacking in the later novel (which actually is a "prequel" to Canticle, by the way).
