First go to Settings > General, then scroll down to Accessibility. Tap Speak Selection, slow down the speaking rate a bit to make Siri’s reading somewhat more natural sounding, and then turn on Highlight Words so that you can see where Siri is reading in the iBook.

Now head over to the iBooks app and open the book you want to hear. Tap the aA icon at the top right of the iBook display, and tap on Themes. Of the three choices, select Scroll. Next, tap a word in the book to select it, then — while holding your finger down on the drag handle — scroll down through the text as far as you want.

Once you’ve selected the text, a menu appears. Tap Speak, and Siri complies by reading your book. Should you need to pause the reading, just select another piece of text and tap on Pause from the menu that appears.