Consistent capitalisation in names.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
Use numbers are keys.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
Use the keys directly since they're integers.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
Pull out BOOK_FILE and save methods.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
Convert keys to integer on load