Trying to find a science fiction book. I think a detective ends up finding a slug-like alien inhabiting a human. Somehow discovers it’s kind and doesn’t want to hurt humans, it had to do it in order to live, possibly after crash landing on earth. Normally they are symbiote with another alien species mutually beneficial. At the end of the book, I think she (detective) lets it into her head to save it. Think sequel?
Stump the Bookseller
New format, same book stumper solving.
Possibly Needle and its sequel, Through the eye of a needle by Hal Clement.
https://en.wikipedia.org/wiki/Needle_(novel)
I think in the sequel, the detective/symbiote is making his host sick through lack of knowledge and needs to track down more member of his species to save his hosts life. I think at the end of the book, the detective/symbiote lets a doctor/symbiote into the host.