The name rang a bell so I searched for them. Looking at the list of books and the covers, I remember reading some of them when I was in high school. And I enjoyed them a lot. From the publishing dates, I think I only read the earlier ones that didn't have "Quiller" in the title.
The search also told me that Adam Hall is a pseudonym for Elleston Trevor. That was a name I did know and associated with these books. I think I read "The Flight of the Phoenix" after seeing the original movie and then moved on to the Quiller series after finding out who the author was.
Not that I am reminded of the books, I remember they were good reads, but know nothing about the later ones. I think I will have to look for them. I have been rereading my Desmond Bagley collection lately. These should do nicely when I finish.