I have the same problem on an iPad Pro 9.7, iPad Air 2, and iPad Mini 4. Tried a complete settings reset and then a restore from iTunes backup followed by turning WiFi off and then on again in the Control Center. This caused iBooks to function again briefly, but upon quitting and restarting iBooks the problem returned.
I have had problems with my iPad Pro 9.7 and 12-inch MacBook that required sending each unit to Apple while both were less than one month old. They replaced the iPad and the keyboard assembly on the MacBook, but I am sorely disappointed in Apple's quality control -- though I am grateful they do stand behind their products.
I hope we don't have to wait long for a fix to this iBooks problem, but I won't hold my breath. Even my close to 30-year-long loyalty to Apple is being sorely tested.