The 'Observer Book of Birds' is one of my most favourite books that I own. I reall love Antique Illustrations of Birds, Mammals and Plants. I think it comes from back in my childhood. My Dad had an old and slightly battered looking Encyclopedia that I would look through when I was little, and there were drawings of all different species with their Latin names.
I often use these images to draw sketches from as I find it really relaxing. I am also in the process of decorating my bathroom with copies of some of my favourites...(pic to follow)