Glassine is a thin, semi-translucent material used by some publishers to cover and protect a book. The material is delicate and easily damaged. These are usually, though not always, unprinted. Some are embossed with a spider web pattern and called Webbed Glassine.
The example shown is on a book dating from 1937. As the image shows, this wrapper is chipped and wrinkled with losses along the edges. It is also somewhat toned and brittle.