When the next novel is completed, the books will form a kind of tetralogy.
In many cases, those books form the spine of a publisher's back list.
Rodriguez has been writing since 1973 and his books form a trilogy.
Together, books and television form a useful alliance which has another set of special characteristics.
These books don't form a trilogy; they have neither character nor setting in common.
Although the books theoretically form a series, actually there is little continuity between volumes.
Together these books form an important archival record and can be seen as early tourist literature.
Your books on crime have formed a foundation for the study of the criminal mind.
I was interviewed for the article many weeks before the final book came out and could not have formed any such opinion.
These books formed a most interesting section of his library.