This is the 3rd book in the "Sew Simple" series by Karin Hellaby.

Attic window is a traditional patchwork block composed of a square or rectangle window with frames on two sides of the square. The fabric strips that create the window are joined with a mitre.

Karin's 3D attic windows technique completely avoids mitred seams, using only triangles to create the windows and frames. She has extended this technique by curving the window sides for an interesting additional effect. If the window calls for traditional sewing techniques like rectangular windows, "fussy cut" windows and quilt borders, she gives the easiest mitre she knows.