Old SF novel series possibly called "Riders"

I am looking to identify a series or at least an individual volume of pulp(ish) sf. Probably, but not definitely, dating from the 1970’s.　I was given the volume in question by an aunt. I was a school boy in the mid 1980s in the north of England. The book was a small format paperback, with a whitish cover.　The blurb made it clear this was part of a series, may be the middle volume of a trilogy but I do not know for certain.

There was an extract from a previous volume that described one man looking out of a window of a spaceship.　In the distance is another ship. Another character asks a question which the first character considers, then when he looks back at the window the other ship is gone. Another detail which is probably significant is that there were aliens, probably but not definitely symbiotic aliens called "Riders". I lost the book before I was a dozen pages in so details are scant.

To sum up:

• Published before 1984
• Soft back pulp sf
• Aliens referred to as "riders"
• At least two volumes, maybe three or more. This was not volume one.