I rated this novel " C+ " when I read it Feb. 16, 1977.
I considered this book, the fourth and last of the series, significantly weaker that the others. Still, most series have trouble maintaining quality / pace / reader consciousness even up to the third book, let alone the fourth. As examples, I cite - well, how much time have we got? Do you want a few examples, or dozens or ...?
The first failed series that comes painfully to mind is riverworld, by Philips Jose Farmer. The initial novel, "To Your Scattered Bodies Go", won a Hugo Award, and had some some very strong points - but even in that introductory segment, there were hints that all might not turn out well. It descended farther and farther with each successive book (perhaps leading to the discovery of black holes?). The final insult - the collection of riverworld stories, ranks with my very very least favourites.
My rating system:
Since Goodreads only allows 1 to 5 stars (no half-stars), you have no option but to be ruthless. I reserve one star for a book that is a BOMB - or poor (equivalent to a letter grade of F, E, or at most D). Progressing upwards, 2 stars is equivalent to C (C -, C or C+), 3 stars (equals B - or B), 4 stars (equals B+ or A -), and 5 stars (equals A or A+). As a result, I maximize my rating space for good books, and don't waste half or more of that rating space on books that are of marginal quality.
I rated The Triumph of Time " C+ ", which translates to a Goodreads score of 2 stars.