We commemorate Madeleine L'Engle with a profile written by her husband for the Horn Book on the occasion of her 1963 Newbery Medal. And we've got some reviews of her work up as well. I like what Ruth Hill Viguers wrote about The Arm of the Starfish; it can be said of L'Engle's work as a whole:
The plot moves with such speed and variety, and emotions are so tautly stretched, that if there are weaknesses, the reader is much too occupied to be aware of them. At the end he might wish that the restraint and subtlety had held to the last page. But the critic who turns back thinking to pinpoint a flaw is caught again not only by the vigor of the plot and the power of the overtones, but by the small imaginative details: apt naming of the characters, realistic conversations, brief moments of awareness of commonplace joys.