The Appendix 2 - Predefined Variables table is very well laid out and easy to glean quick info from.
A big-arse version of this table, with every WALTER variable currently available, set out in titled sections like it is, would be the proverbial DUCKS GUTS.
Maybe add a third column, showing a "common" example of the variable.
(I do realise that it would take an aeon to slap together a tome like that
)