Update: I called wheat paste old-fashioned until a delivery van soaked my cloth book in Golden, Colorado
I was carrying a finished linen-bound journal across the parking lot outside the workshop in Golden when a delivery van blew through a puddle and drenched it, and my bottle of PVA wasn't going to fix a soggy spine by itself. I dug out a bag of wheat starch, cooked up a small batch (roughly 20 minutes of stirring), and re-set the book with that instead, and it dried flat with no stains after about 12 hours under weight. Now I admit the old-timers were onto something, so what ratio of starch to water do you all use for repair jobs like this?