I am fascinated by the description of the pool in the room of stars. I think it's really interesting how there is this pool, it is relatively small and simple (though certainly beautiful), and yet in the reflection of this pool there is "a second deeper sea", the reflection of the stars and the blue painted ceiling. And even more mysterious, when the protagonist dives into the water, he discovers that he is in a third, even larger and deeper sea with rocks and weeds and other inhabitants. It reminds me of C.S. Lewis's description of Narnia in The Last Battle, where Narnia gets larger and larger the deeper in you go.
The biggest part of this chapter though is his discovery of the library, which fills much of the next couple chapters. If discovering the Fairy Palace is the apex of the story, then the books in this library are the heart and soul. Reading these chapters had an effect on me that I could not describe or reproduce with words. Thoughts and emotions alone suffice, yet those cannot be shared in our world here. At least not easily. "... to bathe, with an airy invigorating bath, the limbs which the glow of the burning spirit within had withered no less than the glow of the blazing sun without."
Reading books and experiencing what is described... that makes me think of modern entertainment technology. Movies, and now 3D movies, and Imax, and all of these interactive technologies seem to be trying to recreate what MacDonald has described 150 years ago, a genuinely and deeply interactive experience. It must strike some chord in the human heart that society would spend billions of dollars trying to achieve this.