The Ancient War

"'If you were killed,' she said, 'I'd steal a ship and go hunting for the killer till I found him.'
"'You'd do better to help correct the conditions that led to my being killed.'
"'You're too civilized,' she said bitterly.
"The ancient war, he thought, the immemorial struggle of intelligence to master itself. Nicki could never stay on Earth."
(The Peregrine, Chapter XIV, pp. 127-128)

Over a thousand years previously, an Un-man had identified the old enemy as:

"...man himself...the revolt of a primitive against the unnatural state called civilization and freedom." ("Un-Man" IN The Psychotechnic League, pp. 31-129 AT pp. 125-126)

So has nothing been gained in a millennium? Yes, the ancient war has been won on Earth - and, millennia later, will be won in the rest of the Galaxy.

Trevelyan and Nicki discuss options:

he could join her Nomad ship (this is what later happens);
he might need an assistant on one of his jobs?

Answering that second suggestion, he says that, when he needs an assistant, he gets another Coordinator, usually an otherling. That is the basis on which "The Pirate," written later but set earlier, was built. Reading the Psychotechnic History in chronological order of fictitious events, we find in The Peregrine references to three earlier installments:

the origin of the Nomads;
the colonization of Nerthus;
a mission by Trevelyan and the Reardonite, Smokesmith.