The Secret Visitors by James White

When the World Security Organization asked Doctor Lockhart to treat their mysterious prisoner, they hadn't known that the dying old man would reply to their questions in a totally unknown language. They had expected the stranger to reveal something about the world war which seemed imminent. But they had been thinking in terms of foreign spies - not alien beings!
Now suddenly they found themselves confronted with a Gargantuan task. They had to find a way to another world, a means of communicating with creatures they could barely imagine.
They had to stop a war which was originating in the farthest stars - or else surrender the Earth unconditionally to THE SECRET VISITORS.

Condition: Acceptable. Some creasing on the cover.