I wasn’t as sold on this book as quickly as I was with A Wrinkle in Time, but it grew on me. The final quarter of the book was really strong! (And she didn’t rush the ending, or perhaps she did and I’m just used to it now.)
It’s a bit confusing, but I think I have it sorted out as The Arm of the Starfish is the second book written in the Kairos super-series, the first book of the second generation O’Keefe Family series, and the fourth book chronologically in plot line. Looking at the publication dates, it looks like L’Engle bounced back and forth between the two series (and another one) while she was writing in the ’60s and ’70s.