This is pretty simple. You just lay some new glue on the spine and cover, let it dry a bit, and then stick it on. I bought this glue called “padding compound” which is a special rubbery glue that is used to make notepads. It’s more flexible than white glue.
They sell it to printers (who make pads of carbonless forms, and things like that). Normally, they’re sold by the gallon to the trade, but some people sell small bottles on Ebay. If you don’t care about flexibility, just use white glue.