As I recall the first two books are pretty good, after that the novel series kind of goes off the rails.

As I recall, the original novel is vintage Clarke of the better kind, and then the rest are [co-]authored by some entity named Gentry Lee and basically unreadable.

You are right... In this case I think it might be a good example of how serious scientists are not always the best fiction writers.

https://en.wikipedia.org/wiki/Gentry_Lee

Shame on me. I never knew, and I never bothered to look it up.

It is not an unreasonable assumption to make, there are a lot of novel series where the 3rd, 4th, 5th novels are ghostwritten by some third party hired by a publishing company that owns the rights, to cash in on a well known novel's name and author.

The first book is certainly very different from the rest. If not for other reasons then simply because it is a self-standing short novel while later books tell a continuous story spanning three books, with each book being about 2-3 times longer than the first one. Calling them unreadable is a bit harsh though. I still found them to be good sci-fi reading.