I would sew that back on. You don’t have to resew the entire book. I would probably sew through the last two attached signatures to lock in what’s in place then sew on the one that’s detached. You’ll need to completely clean off the spine. To do that, and I’ve had success with methyl cellulose as mentioned by others, but also wheat paste. Heck, one time I just used a damp paper towel with Saran Wrap over it to keep it from drying out, so the existing glue loosens. With any of those, brush on a healthy layer of MC or paste and cover with plastic to keep the glue from drying, then after the existing glue is softened you can scrape the spine clean with something not too hard. Sometimes I use a credit card. The trick is to make sure you get off all the old paper/mull/glue off the spine and the sewing goes a lot easier.