I remember very little about this book, but I remember it featured a lot of eerie black and white pencil illustrations. I think it was about a young girl who is kidnapped by people who live under the hills. Its the story of her stay with them and how she escapes. I remember the illustration of a woman watching her from the woods. Any advice on what book this is?
Could it possibly be The Perilous Gard, by Elizabeth Marie Pope? The story is set in early Elizabethan England, and the main character, Kate Sutton, is given to the Old Ones after she learns of their existence. The story is based loosely on the story of Tam Lin, in which she saves her love interest from being sacrificed as a teind to the old gods. The version I have is only a paperback without illustrations, but it is a later printing; the original 1971 edition may have had pictures.