It can be delightful to root through obscure corners of the Library of Congress website and unearth something special such as a high resolution full book scan of a beautifully illustrated volume. This self-portrait of William Blake comes from Songs of Innocence and Experience, published 1794. He's an important painter and poet; Wikipedia ought to have a featured portrait.
Every restoration has its challenges. The main challenge in this one is that the aged paper does not lie flat. Scanner light is highly directional so digitization enhances small buckles in the original image. You'll see this as a series of bright and dark horizontal bands near his forehead, chin, and chest. The example below is an interim save after initial dirt and scratch removal.
Distracting, isn't it? The brightness gradient tool can be used here, but it only helps a little bit. The page itself is slightly brighter at the lower left corner. So one quick gradient adjustment at a very slight setting is a first step toward correcting the problems. In this instance I used about 2% opacity. Most of these issues have to be solved another way.
The solution here is to look at each buckled area in terms of what it changes about the image: brighter above, darker below. This can be fixed incrementally with multiple feathered mask selections in a series of brightness adjustment layers: brighten the dark sections and darken the light sections. The trick is to make plenty of layers, draw each one separately, and to change just a little bit at a time. This restoration went through 30 adjustment layers, with feathering staring at 40 pixels and decreasing to 25 and 10 pixels, nudging the brightness about two notches at a time (sometimes one and never more than three). The direct result of that follows:
That's more like it: almost like taking a gentle iron and pressing the paper flat (which of course we don't dare do to the actual physical copy). The final restoration added only moderate changes to curves and color balance.