This book is quite unusual among the Chronicles of Narnia.

It's trippy.

Warning: major spoilers

Voyage of the Dawn Treader retains the allegorical crypto-Christian universe of the other Narnia books, as well as the standard structure: the kids go along on a big adventure, overcoming many dangers with courage, faith, and dinvine intervention, and in the end Aslan tells everyone the moral. However, the other books seem mostly concerned with ethical behavior, self-cultivation, and the transformative/redemptive power of Christ. The difference is that in Voyage, Aslan doesn't come down to help the benighted humans -- the humans go up to him.

This is the mystical tradition in Christianity, the active reaching out towards God. Instead of needing divine assistance with their worldly affairs, the characters turn their backs on the world and directly pursue the sacred in abstract form. They're clawing their way back up the tree of life! (to steal someone else's metaphor). As they move further into the sacred, they become purified -- they have the peace and the far sight that comes with enlightenment. Note that as they become farther from land (i.e., less "grounded") they also become less bound to the material world, subsisting on energy rather than matter. To see other versions of this archetype, compare the ending of A Wizard of Earthsea, or the scenes in Lord of the Rings when the characters are living off lembas.

People familiar with the early psychedelics movement in the 1960s will recognize the clear white light!

The ending can be taken as a rejection of pure hermeticism. Caspian desperately wants to stay in the presence of the divine, but he is given the harder path of integrating his mystical experience with the real world, and working to help create the Kingdom of Heaven. Reepicheep gets to experience the final ascent, but he also dies*. Lewis definitely believed that we have a duty to others -- one which God wants us to fulfill, not shirk through ecstatic navel-gazing.

Of all the Narnia books, this one does the most to recognize the sacred in humanity. God is not something that comes down and saves us; it's something that is within us, something we can directly experience. This is a nice counterpoint to the other stories, which cast Aslan as a bit of a superhero.
* Maiessa points out that Repicheep doesn't so much die as he literally and physically enters Aslan's father's kingdom. I think the point is moot, since it is later confirmed that he has gone into the afterlife, and he's separated from the affairs of the living in the same way that dead people are. However, it is significant in that he does it willingly, and no one acts or feels as if he's died.