A Wrinkle in Time
The mysterious Mrs. Whatsit, Mrs. Who and Mrs. Which send Meg and Charles Wallace through time and space to rescue their father on the planet Camazotz, accompanied by their new friend Calvin. Along the way, the three children learn about the "Black Thing", a cloud of evil that shadows many planets, including Earth. They encounter a Brain named IT, which controls the minds of people.
A Wind in the Door
Meg, Calvin and the disagreeable school principal Mr. Jenkins have to travel inside one of Charles Wallace's mitochondria to save him from a deadly disease, part of a cosmic battle against the evil Echthroi and the forces of "Unnaming".
A Swiftly Tilting Planet
Charles Wallace must save the world from nuclear war by going back in time and changing might-have-beens, accompanied in spirit (through kything) by Meg at home.
alibris.com and wikipedia
eBook The Time Trilogy (A Wrinkle in Time Quintet, #1-3)