iPad pro sleeps when using iBooks

This isn't going to be at all helpful for you, but all of my devices over time (5 iPhones, 3 iPads) have always gone to sleep in iBooks after whatever amount of time I have Auto-Lock set to. How have you previously made your iPads ignore the Auto-Lock time? Was there a setting you chose? It might be dangerous for me to use it, because I'd end up falling asleep reading and waking up to a dead iPad, but it would be good to know how to do it—and I'm annoyed I've never come across it.

Thanks!