The second book in the series is A Wind in the Door. Somehow, that was the only book in the series that I owned as a child. Like most books I owned, I read that one over and over again and know it better than the rest. In this one, Charles Wallace is quite ill, due to a battle being fought in his mitochondria. Meg and Calvin end up traveling inside of Charles Wallace, with some other supernatural creatures, to fight for his life and the fate of the galaxy.
There are more books in this series. I have read them only once. There is also a spin-off series that features the O'Keefe children. This is another series that my friends and I devoured in elementary school.