There is no evidence in the text of this book that it is SF. Yet here I have a copy, published in the SF Masterworks series.
Graham Sleight addresses this in his introduction, but doesn’t try to give a conclusive reading either. There is no definitive answer, as the work is deliberately ambiguous.
The titular Sarah is a woman described as ‘ugly’ who turns up in the camp of some Chinese men who are working on railroads in the USA of the 1870s. She speaks no known human language, though she does make sounds. She gains her name later because, a character says, ‘she sings like an angel’. One of the men, a young man called Chin, is volunteered to try to find where she belongs, or failing that, at least get rid of her, so she stops distracting them.
So begins a trek across the Pacific Northwest and beyond. Along the ways we meet various characters with various good and bad qualities.
The ending is, as I say, ambiguous. We never find out who or what Sarah Canary is. But the journey is quite enjoyable.