Recovering and... recovering.
I've only ever made new bindings and repaired some existing books with basic repairs (I'm certainly not qualified for actual conservation work). I never recovered an already made textblock before. My youngest daughter's best friend had a treasured book that she asked me to fix. The cover was really beyond repair. I removed the text block, gave it a little spine reinforcement, and made a new case. The book was about a wolf, so I debossed the silhouette of a howling wolf into the front cover (with two different bone folders because my styluses were missing and of course showed up right after I finished this book). I didn't want to throw away the boards from the original cover though; I trimmed away the fraying parts and made a simple Coptic style sketchbook for bestie. I have to say that recovering a home for the original boards was considerably more fun than making a new cover for the old text block.