With three fingers, tap the screen two times, or three times. Those gestures toggle VoiceOver speech and the screen curtain (winch makes the screen black). Alternatively, just hold the power button for ages and restart your iPad.

For future reference, what you turned on is called Voiceover. When Voiceover is on, you must double tap to select items. When you double tap, it will select the last thing Voiceover said. Speak screen or speech selected text might work better for you in the situation you've described. You can still find these features in your accessibility menu, but they are under speech. Speak screen speaks the information on your screen when you swipe down with two fingers from the top of your screen. Speech selection adds a speak option to the menu of highlighted text.

I like the fact that you thought to come here; that's smart. I think what you want is an option called Speak Screen. It's in accessibility settings under speech. Once you turn it on, you can swipe down with two fingers or tell siri to "speak screen", and your textbook will be read to you without VoiceOver taking over the whole device. It's intended for exactly what you want to do, rather than control of the whole OS.

Additionally, if you want a brief tutorial or practice on how to navigate your iPad with these commands you can double tap the screen with four fingers. This opens up a practice mode in which you can experiment with swipes and presses of 1 to3 and even four fingers and learn a little bit more about how to control the iPad in VoiceOver mode

Or tap screen 3 times

Try pressing home button 3 times.