Blink, and a simple trilogy expands to four or more parts, a sequel pops up to a book written years before, or a series suddenly develops a new name. The series phenomenon is not confined to SF and fantasy, nor to a lesser extent, horror.
Series exist in detective, western, historical and romance fiction.
Certainly the popularity of Tolkien in paperback in the late sixties contributed to a stronger interest in fantasy writing, but the enormous commercial success of Terry Brooks The Sword of Shannara in 1977 paved the way for a demand for more series in science fiction and fantasy. The first edition of The Four-Part Trilogy was published by Gaslight Books in book form in 1993. Omnibus books that collect two or more titles together in one physical volume are included if it could be ascertained that they were commercially distributed through the book trade. Many omnibus editions were published only by the US Science Fiction Book Club and only available to members residing on the North American continent. Some copies filter their way to Australia, but on the whole they are difficult to obtain in this country.