We use Managed Apple IDs and I have pushed out a few books. Since books are assigned to the user rather than the device you need to make sure that the Apple IDs have accepted the VPP invitation. Under the VPP option in Settings we have the Automatically register with volume purchasing if users have Managed Apple IDs option checked. Then we had to create an invitation in the Users section for that VPP option. I wish I could pull screenshots but since we are on the cloud it times out trying to get to these pages anymore.
Once that was done I could purchase the books from Apple School Manager and have the licenses available in the Volume Assignments section under Users. There you can scope it to your groups.
