These five books comprise an imaginary pentalogy of Science Fiction books, all having a cybernetic/medieval fantasy bent.
Here's what I imagine happens in the books:

Book One introduces us to the main character(s) of the pentalogy. These characters become aware of something(s) called the "Devastation Angels," a nasty groups of something-or-other that must be stopped, else something truly "devastating" will happen in their country/world/solar system/etc.

Books Two through Four will feature various steps along the road to defeating/destroying/preventing the release of the Angels. In each book, we will be introduced to someone/thing new that will help/hinder our intrepid band in their quest. The end of each book will seem like a victory for our fearless team, but of course, there will be more perils to face in the next book (stay tuned!).

Book Five will reveal that, despite the best efforts and sacrifices of our beloved comrades, the Angels will, in fact, be unleashed. The resulting conflict -- involving as many of the characters from the previous four books as we can reasonably cram in/resurrect from the dead/summon forth as deus ex machina -- will be costly and epic, but eventually justice/harmony/profitability will be restored, everyone will learn a valuable lesson, and the boy/girl will get the girl/boy.

I designed all the covers with their vertical symmetry (okay, okay, it's a habit of mine!), and the corresponding border, along with the re-use of the "Devastation Angels" logo (from the first book), as a means of establishing an "identity" for the series. I'm not sure that that it is immediately apparent...:)
Children of the 60s, or very early 70s will recognize the "pun" of the imaginary author's name.