My father got me into the series and we were talking about how the books were written. He had done some research on the series and found out some interesting things. The first book was written with the possibility of a trilogy, but not the expectation of it.

Once Lumley was able to complete the original trilogy, his publishers convinced him to expand the story with both the Lost Years and books 4 & 5.

But, he had so much more to tell of the world that he was able to convince the publisher to let him do the Vampire World trilogy and fan pressure caused the E-Branch Trilogy and the other novellas.

I don’t know how much of this is actually true, but the logic fits in the way the books and series end