UPDATE: Mabel found it! She remembers it being Barbara Michaels' The Dark on the Other Side! Hooray for the book obsessed with long memories.
I've been searching for a particular book for a while and I think I'm getting close to finding it. I thought the author might have been Mary Stewart, but now it's been suggested that it might be Barbara Michaels, and having checked out her title list, it's sounding possible.
This is a book I read in middle school, in the late 1970s I suppose. We were reading Mary Stewart, H. Beam Piper and Nancy Springer at the time, so it would fit in with that genre and reading level. The only thing I remember is that it involved a couple and a third person, another man, and that the last line of the book was "It was a silver bullet." This referred to the fact that one character had shot another, a suspected werewolf, and wanted to ensure that he successfully ended the terror of the story.
I looked at the synopses of several Michaels titles, and they sound quite familiar, so I think that even though I had totally forgotten she was a favorite author back then, it's likely. Still, I can't pinpoint the exact title. Sons of the Wolf doesn't look quite right, but if it fits the rest of what I have written, it may be the book I'm looking for. If any Thingamabrarians or others reading this can identify the book, I will be quite grateful. Otherwise I may just have to wait until I can go to the public library, find everything they've got on the shelves by Barbara Michaels, and check the last pages of each volume!