Progo is straight out of Ezekiel, and L'Engle is at her most consciously imitative of C.S. Lewis here, this book most closely resembles his That Hideous Strength in its suggestion that scientific phenomena are presentations of supernatural events - think of his presentation of angels as "macrobes" there.

Next up is A Swiftly Tilting Planet. It's Charles Wallace's show, and he's old enough to be interesting beyond his prior existence as a wunderkind. It is the trippiest book of the first four. The series was not yet a quintet when I last read them.