The Rings of Tantalus — Edmund Cooper

First they went to Kratos - and faced the Deathworms. Then they went on to Tantalus and tried to fathom the Rings - which could only have been fabricated with the help of highly sophisticated machinery.

And then they discover an alien spaceship orbiting the planet in strict silence and incommunicability.
When they finally boarded the vessel, it was obvious that it had been derelict for centuries - as a result of some dreadful battle. Meanwhile down below, the enigmatic Rings lay waiting to deal with intruders - as they did the deadly vampire tree. Once again, Commander James Conrad and his team of Expendables faced their mission to find planets suitable for colonization.

The second of a new series of books, The Rings of Tantalus seems guaranteed to further launch Richard Avery as a very special writer of modern science fiction novels. And there are more to come.