I gave it another try by just adding 2 additional loops on each side withing the base plane. Then i switched to orthographic front/side/top views and just selected the bottom right vertex. If you activated proportional editing, use your Mousewheel to adjust the circle of influence (white circle). I also did some manual editing with Edge Slide on a few vertices. Between that, i selected some of the top and right vertices and pressed H to hide them, so no operation will affect them. To bring them back, press Alt+H. To keep all outer edges sharp and have them not affected by the Subdivision modifier, crease all edges with a value by 1. To keep them sharp and untouched when you edit the cover, Hide (H) them.
I increased the subdivision one time to see if it allows for better results, but there is already enough topology within 3 steps. If some vertex goes out of X or Y space and cause a crease, manually bring it back in line with activated Vertex Snapping to one of the most outer verticies.
page_bend.blend (553.4 KB)
Most perfect results can be archived, when the cover is unwrapped and a vertical stripes texture is added.