Finding Prejudice Everywhere

In addition to the sexism found everywhere in Lewis, including Narnia, there is also the accusation of "racism*" in The Horse and His Boy. This is incredibly superficial. Lewis draws an enormous amount of his material in that book from 1001 Nights, so if you don't like the depiction of the residents of Calormen, you might go after that author first. Next, the Calormens believe in gods, and Islam is (arguably) even more strictly monotheistic than Christianity and its many saints, sometimes celebrated with an ambiguity that makes Reformed and Anabaptist and Pietist and Quaker churches uncomfortable. It's not just Catholics.  It's Eastern Orthodox, Copts, Lutherans, Anglicans, and probably some groups I am neglecting to mention. 

More subtly, both Lewis and Tolkien (and Williams and McDonald and the hundred derivative authors) have been accused of a Christianity too tinged with Northernness, of Germanic and Celtic creatures and spirits, of world-endings that have too much Gotterdammerung.  It is a bit ironic that someone who is faulted for being too Northern is faulted for not being Southern enough. Modern people want to hear the exact words they want to hear about whether Islam is correct, much as various stripes of Fundamentalist want to hear their precise salvation vocabulary. Essentially, they want to comfortably put things in a box so they don't have to worry about it anymore and they can go on to the political/cultural wars, which may be their Real Gods.

One might be able to say that by The Last Battle the issues are more clearly Islam/Christian, but even there it's not solid. It may be the syncretism (which Moslems would also reject; see persecution of Bahai) Lewis regarded it as a heresy, as Buddhism is a Hindu heresy, even if it may be more "advanced" in our conception, which is different from being flat wrong. Lewis believed other religions were right in part. To the question of whether YHWH and Allah are the same person your might find an answer that you like elsewhere, but in Lewis the answer is that YHWH is YHWH, and Allah is something like that, from which some might learn, and even Christians should have a glance at.  You aren't going to a yes/no that is clearer from him.

One more thing.  In Narnia, the most sexist males are the worst characters, and it is one more irony that both Rabadash, the various Calormen males, and even Shasta (whose sexist attitudes are contrasted nicely with Aravis's classism) get their comeuppance for it. It is said in American that you can have rights for women or multiculturalism, but you can't have both, and Lewis illustrated that half a century before that was first said. In the end, to accuse Lewis of racism on this is to reveal one's own racism, that all is judged by darkness of skin, which we have to reflexively support, as opposed to lightness of skin, same.

*Last I checked, even in my movie-going era Omar Sharif was considered white.  Exotic might be the term, but still very much white. Thus I describe this supposed prejudice as "anti-Muslim," or "anti-Arab" rather than by race per se in discussing it now. And even then, the Persians might bristle at the "Arab" part.