Fix some subtitles and remove books shortcut
Fix some subtitles and remove books shortcut

# There are now two versions of the A-Level book, and three other books.
# We need a generic book page to link to, not this.

ucam-cl-dtg/isaac-app,ucam-cl-dtg/isaac-app,ucam-cl-dtg/isaac-app,ucam-cl-dtg/isaac-app