Everyone who started one, read the entire series. A Great New Solar Queen Adventure Andre Norton - Solar Queen stories among my all time favorites. I doesn't matter. Don't let their classification throw you off. They are good books.
Magic takes the place of science.
The Healer is an explorer who was forced into symbiotic sharing of his body with an alien organism. The symbiot has biological control at a cellular level and has stopped aging making the explorer change identities and careers over and over as the ages pass. The dual mind makes the pair best able to respond when humanity is psychically attacked. Hal Clement's Needle has a similar theme. A juvenile SF with the symbiot alien detective merging with a teenage boy to help find an escaped member of his race hiding in a human.