Published eleven years after A Wrinkle in Time, A Wind in the Door represents, in my opinion, an improvement on the first Time Quartet novel. Although A Wrinkle in Time and A Swiftly Tilting Planet cut this book close, I rate A Wind in the Door higher because it has everything good that A Wrinkle in Time has, but it has more. And unlike A Swiftly Tilting Planet (which is better written still), Meg Murry is again the protagonist of this story.
It is two years after the events of A Wrinkle in Time. Meg Murry is now in the same high school as Calvin O'Keefe, and in a bit of denial over the full extent of her feelings for him. Charles Wallace has entered elementary school, and is having a difficult time of things. He is being bullied, and there appears to be little that his teacher or the school's principle (Mr. Jenkins, the same principle that Meg had a number of run-ins with during the time of A Wrinkle in Time). On top of all this, he is coming down with a mysterious illness that the doctors can't diagnose. When it revealed that the same forces behind A Wrinkle in Time's Black Thing are behind Charles Wallace's illness, Meg, Calvin, Mr. Jenkins and a cherubin named Proginoskes must perform their own Fantastic Journey and shrink down inside one of Charles Wallace's mitochondria to save his life. Through the use of kything (like telepathy, only more -- rather than a sharing of thoughts, kything is a sharing of being) and the power of naming things, they fight the Echthroi (the forces of darkness).
As we are now familiar with the characters of Meg, Calvin and Charles, and we slip into their crisis all the more easily. Meg and Calvin's relationship is taken further (after the complete intimacy of kything, who else but each other can they end up with for the rest of their lives?) and Meg learns more about the true power of love. And with Charles sick, the stakes are even more personal than Meg's vanished father in A Wrinkle in Time.
The core of the Time Quartet (or Quintet, depending upon your view of how An Acceptable Time should be placed) is very much made up of A Wrinkle in Time and A Wind in the Door. The second builds up so much from the first so well that, if you read only two books by Madeleine L'Engle, "A Wrinkle of Time" should be your first, and "Wind in the Door" your second.