These are coming out of edx-platform, unused in years

https://github.com/edx/edx-platform/pull/15830

CHANGELOG.md

@@ -376,3 +376,4 @@
 
 - Role: edxapp
   - Added `EDXAPP_VIDEO_TRANSCRIPTS_SETTINGS` to configure S3-backed video transcripts.
+  - Removed unused `EDXAPP_BOOK_URL` setting

playbooks/roles/edxapp/defaults/main.yml

@@ -270,7 +270,6 @@ EDXAPP_FEATURES:
   SHOW_HEADER_LANGUAGE_SELECTOR: "{{ EDXAPP_SHOW_HEADER_LANGUAGE_SELECTOR }}"
   SHOW_FOOTER_LANGUAGE_SELECTOR: "{{ EDXAPP_SHOW_FOOTER_LANGUAGE_SELECTOR }}"
 
-EDXAPP_BOOK_URL: ""
 # This needs to be set to localhost
 # if xqueue is run on the same server
 # as the lms (it's sent in the request)
@@ -971,7 +970,6 @@ generic_env_config:  &edxapp_generic_env
   LMS_BASE: "{{ EDXAPP_LMS_BASE }}"
   CMS_BASE: "{{ EDXAPP_CMS_BASE }}"
   LMS_ROOT_URL: "{{ EDXAPP_LMS_ROOT_URL }}"
-  BOOK_URL:  "{{ EDXAPP_BOOK_URL }}"
   PARTNER_SUPPORT_EMAIL: "{{ EDXAPP_PARTNER_SUPPORT_EMAIL }}"
   PLATFORM_NAME: "{{ EDXAPP_PLATFORM_NAME }}"
   STUDIO_NAME: "{{ EDXAPP_STUDIO_NAME }}"