I've read all the Chronicles of Narnia books many times over, and I highly recommend them. When I heard people claiming that the books were a Christian allegory, I was actually pretty surprised. There are dozens of references to all kinds of religions, cultural myths, and belief systems apart from Christianity.
Some of them are actually very far removed from Christianity, stemming from Buddhism, Ancient Egypt, and even some lesser-known tribal beliefs. In fact, a lot of Christian and Evangelist book reviews have stated the series is too heavy in the occult. In my research, Lewis himself refuted claims that they were written as a Christian allegory. He says he wrote them with the intention of answering the question "What if there was another world that Jesus decided to save." In writing the Chronicles of Narnia, Lewis says he used symbolism to answer that question.
It is compared to other stories inspired by the "what if" question - "what if the main characters of
Hamlet were lions? What if Abraham Lincon hunted vampires?" The same question a lot of writers ask when coming up with a story idea. Some interesting information from Lewis, the actual definition of allegory, comparisons to other literature, and thoughts on his writing process about the Narnia books can be read
here
There's no denying that there is Christian symbolism in the books, but there is also symbolism for a lot of other religions and beliefs as well. Maybe Christianity is the most prevalent, but I have trouble lumping all the books into the "Christian allegory" group when there is so much more going on that is unrelated.