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

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.