I would say that her two outstanding books are The Human Evasion and Advice to Clever Children. The first book, while still very obscure, has slowly acquired a fanbase online; but the second book remains thoroughly unknown.

Single data point: I had heard of (and read a few pages of) the second of those but not the first.

[EDITED to add:] Second data point of a very different kind: Google gets about 3x more hits for <<"advice to clever children" celia>> than for <<"the human evasion" celia>>.