A trace of memory
Description from [goodreads][1]: 

> When Legion meets a strange old millionaire called “Foster,” he doesn't know that his whole life will never be the same again. For “Foster” has a strange story to tell. Part of his story is contained in his diary, which not only describes things in the very recent past, but goes back for centuries. . .
> 
> Foster has hired Legion to help him regain his lost memory. Then both men are suddenly attacked by alien powers. They flee to Stonehenge in England where they discover a strange control chamber deep in the ruins. 
>
> They succeed in bringing down to earth an alien space ship which has been in orbit for thousands of years. Eventually they both become captives. . . on another planet.


  [1]: https://www.goodreads.com/book/show/1549260.A_Trace_of_Memory
Fiction, science fiction, general
Keith Laumer