I read James Kahn's first book, World Enough and Time, sometime in the mid or late 80s, borrowed from a relative. I don't recall why, but somehow I had the impression that it was a classic. I read the sequel, Time's Dark Laughter a few years later. Even though I wasn't really that impressed with the books, they've stuck in my mind as wrongly overlooked masterpieces (wrongly overlooked interesting books, anyway).
When I saw this newer book on NetGalley, my first reaction was "Huh. Very similar to that other guy's books." Reading a little further, I thought "Wait! Same guy? New sequel? Exciting!" Of course, it turns out that this sequel actually came out in the '80s; I just hadn't noticed it. Kahn has recently reissued them as ebooks, with some new material.
I couldn't remember much about the first two books, even with the help of Goodreads, Wikipedia, and other aids. Nonetheless, I started this book with some anticipation.
Timefall works as a standalone book. It eventually refers pretty heavily to events earlier in the series, but none of it is hard to decipher, and the early portion is self-explanatory. Kahn himself comes in in extensive fore- and afterwords to provide some structure (I presume this is part of the new material).
Unfortunately, the book is also just not very good. It's certainly not the exciting discovery I had anticipated. Instead, it's a fairly standard jungle jewel adventure with some time travel mixed in. It's fast-paced easy reading, but the characterization leaves something to be desired, and there's a lot that happens for authorial convenience. It's written for the Indiana Jones period - not surprising, since Kahn wrote the novelization for one of the Indiana Jones films not long earlier.
The book careens along in an adventure sort of way until suddenly the previous books turn up, in strange twists that, while they aren't confusing, don't really seem related to the story. I think of it as: California, jungle, jungle, adventure, jungle, vague and complex metaphysical risk to existence of the entire universe, jungle, adventure, weird metaphysics, California. In the end, I could comprehend what was happening, but none of it really made very much sense. Two key actors are introduced and then dropped completely. The solutions to the end of the universe turn out to be a) really obvious, and b) brow-furrowingly "wait, I thought it had to be..." The actual saving of the universe is left as a problem for the reader.
All in all, disappointing. Separate from that, it's just not a book I can recommend. If you read the first two books, loved them, and have them firmly in mind, go to it. Otherwise, nostalgia is better in the imagination; visits to old haunts are seldom satisfying.