I loved "A Wrinkle in Time." And I was fascinated by "A Wind in the Door." When I first read it, at age 11, I made the mistake of picking it up at midnight. I didn't get to sleep until I finished it, around three in the morning.
But "A Swiftly Tilting Planet," which is much more fantastic than the first two of the series, remains my favorite of the trilogy. It is complex without seeming contrived or overly simplistic (as the first two occasionally felt). The characters are fully human, complete with flaws and faults--even the angelic (or is it Christ-like?) Charles Wallace.
Charles travels through time with the unicorn Gaudior, at each period going Within a person, becoming that person, feeling as he feels and thinking as he thinks. As Charles Wallace, Within a person, is absorbed into that person's conscious, so do I become engrossed in that person's story--it's painful when Charles comes out.
My favorite of the people Charles goes Within is Matthew Maddox, the young writer. Every time I read the novel, I wish yet again that he and his books were real.
I was initially disappointed that Meg, who is so brilliant at math and the sciences, appears to have done very little with her abilities. But nowhere does the book state that she has done nothing; and in fact, it would not be inconsistent with Meg's character to have done less than would be expected of her. Meg has always doubted her abilities, and she has always felt inferior to her mother. Self-doubt is a terribly crippling thing, and I myself can testify that being the daughter of a brilliant mother can be very disheartening--how can you ever measure up to her standard?
A side note: The structure of Italo Calvino's "If on a winter's night a traveler" reminded me vaguely of this novel. It's not a perfect parallel, but if you're an older reader and you liked "A Swiftly Tilting Planet," you might try the Calvino; it's quite interesting.