Public Library downloads
I think what you are asking is how to turn off the Shuffle.


Navigate to the “Now Playing card. You can access the “Now Playing” card from anywhere by swiping up from the bottom of the screen to bring up the Control Center, then swiping left.
Select the name of the song (Audio Book)
select the crossed arrows to toggle the Shuffle setting. The arrows will show with a darker box when enabled.





  To the right of the Shuffle icon is the repeat icon. Tap it to repeat
  the current playlist. Tap it again and a number 1 will appear next to
  it and the device will repeat the current song. You can also drag the
  ball on the song timeline to skip forward or backward on the current
  track. This feature comes in handy if you’re listening to an audio
  book or a long compilation.


Source:

I little trick I use is to add a number (1, 2, 3...) in front of the chapter (file name) so to keep them in order.