Red threads is Rex Stout writing pre-Nero Wolfe (his most famous detective). This mystery is pretty good and involves a strong American Indian theme, some of it believable and some fairly ridiculous-sounding. I suspect Mr. Stout did scant research- or perhaps he just didn't have good research materials available to him.
The romance and characters are the strong points in the book. Although even the characters in this early Stout book can seem a bit over-done.