diff --git a/scripts/calibre-install-unstable.sh b/scripts/calibre-install-unstable.sh
index bb2af7383..972c4a2e3 100755
--- a/scripts/calibre-install-unstable.sh
+++ b/scripts/calibre-install-unstable.sh
@@ -4,13 +4,17 @@
 # breakthrough possible!
 #
 # Calibre 3.23 is the latest available from testing as of 2018-05-10:
-# https://packages.debian.org/search?keywords=calibre
-# (SEE ALSO http://deb.debian.org/debian/pool/main/c/calibre/ ?)
+#
+#   https://packages.debian.org/search?keywords=calibre
+#   http://deb.debian.org/debian/pool/main/c/calibre/
+#   http://raspbian.raspberrypi.org/raspbian/pool/main/c/calibre/
+#   http://archive.raspbian.org/raspbian/pool/main/c/calibre/
 #
 # Might break future updates; you have been warned.
 
 export DEBIAN_FRONTEND=noninteractive
-# Updates to calibre & calibre-bin to "very latest" 3.x from unstable
+# Prepare to update to latest from unstable
+apt-key adv --recv-key --keyserver keyserver.ubuntu.com 7638D0442B90D010
 echo "deb http://deb.debian.org/debian unstable main" >> /etc/apt/sources.list.d/unstable.list
 apt update
 apt -y install calibre calibre-bin