This beloved book was not in good shape. Most of the pages needed some amount of paper mending and guarding, and the sewing unraveled as I started working on it. 
I gave up on trying to salvage the sewing and un-did it completely. After numbering the pages, I separated the folios and mended the tears. Then I resewed the entire book.
I built a new spine with blue cloth since the remnant from the original spine is also blue.
I consolidated the frayed corners with paste but did not cover them. I thought they should retain the look of their age and wear, their roundedness.