There was a book series I found in my school library, based on several unrelated Robotech animes, that combined them all as a continuous timeline. In total I think it was roughly 20 books total- probably the longest by Volume count that I've read
I enjoyed the "Breakers" series of books by Edward W. Robinson. It's a fun post-apocalyptic/alien invasion story with lots of interesting characters and some occasionally very funny dialogue.
The Kurtherian Gambit interrelated series of novels. 20 books in the main series and numerous offshoots and secondary series that tell other stories in the same universe.
Comments
It’s ~41 books, I think I got up to 20 or so
My favorites were:
The Sword of Shannara (1977)
The Elfstones of Shannara (1982)
The Wishsong of Shannara (1985)
Also The Druid of Shannara (1991)
Keith Parkinson did the cover art for several of the early books.