I enjoyed the first three books in this series when I was younger. The fourth book, Many Waters, I read more recently, and it was good, if not with the same spark as the earlier ones. The fifth book, unfortunately, doesn't stand up.
An Acceptable Time is about Polly, a teenager staying with her grandparents. The rationale for the plot is hinted at but never really explained.
It's a pleasant enough read in some ways, but much of it seems haphazard. The plot seems more of a sketch than a final product, full of inconsistencies and lucky coincidences. The science is decorative but vague and not very logical. Religion obtrudes more awkwardly than in the previous books. Characters are black or white, and fairly flat. At the same time, the tone is light and fun, and some of the animal characters are nice companions.
All in all, a disappointing book. Fans of L'Engle and her various interconnected series will no doubt want to read this. For others, I advise stopping after A Swiftly Tilting Planet.