Possible bugs and improvements
Could you clarify how it is supposed to work? I added "Read" and "To Read" lists. I created 2 custom columns. And it seems that all 3 lists (including "Default") share the same column and tags (to add/remove). Every time I change either the column or the tags in the plugin properties all lists are affected, not just the list shown in the "Lists" menu.
It would be nice if each list has own "Tags to add/remove when adding to the list" value. It will allow to display different text for different lists. The idea is that when all columns are displayed in GUI it easy visible what lists some book belong to (without looking for the columns names).
What types of columns are shown in the "Column to update" menu? It seems that only the "Comma separated text, like tags, shown in the tag browser". I did not try all possible types but I added "Text, but with a fixed set of permitted values" column. The column is not shown in the menu.
Some custom columns can be displayed using colors. Rating columns are displayed as starts. It would be nice if such columns can be used in the reading lists.