How do I keep the text in the vertical mode while reading ibooks?

Hello yramnelle,

I also like to lock the screen when reading.

You can lock the screen rotation with Control Center or maybe the side switch depending on your settings.

Control Center - iPad User Guide

Control Center

Control Center gives you instant access to the camera, calculator, AirPlay, and other handy features. You can also adjust the brightness, lock the screen in portrait orientation, turn wireless services on or off, and turn on AirDrop. For more information about AirDrop, see AirDrop.

Open Control Center. Swipe up from the bottom edge of any screen (even the Lock screen).

Screen Rotation Lock button:

Volume buttons and the Side Switch - iPad User Guide

Use the Side Switch to silence audio alerts and notifications or to prevent iPad from switching between portrait and landscape orientation. (On iPad models without a side switch, use Control Center.)

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Nubz