Scottish Traditional Recipes, by Christopher Trotter, is one such volume. It's true that its main focus is on fish, meat, poultry and game (venison and salmon both make a strong appearance). But more than a few of its 130-odd recipes are either naturally vegetarian or can easily be adapted. The Vegetables and Grains chapter, for example, includes a barley risotto (but you need to use vegetable stock rather than chicken stock) and colcannon (which I always thought was from Ireland, but which has Aberdonian connections, according to the author).