Keep play books audio in the background?
According to this google fourm's message, 17 Dec 16, this is a feature since version 3.11.19

Quote danjaf
Confirm this has been resolved in version 3.11.19
I am running Marshmallow 6.0.1
Thank you to the developer who heard our first world problems.

I have tested myself in at the current version of 4.4.9_RC07.213889590, released 21 September 2018, and can confirm it is still present.