The most active members of LtU for the most part already have a solid foundation in logic. For the rest of us interested in language design, but who are not already logicians here is a brief reading list on logic, focusing on natural deduction, the preferred method of expressing type systems.

More experienced members of LtU may perhaps consider contributing to the discussion with comments on the suggested reading or alternative suggestions.

[Edit: removed A History of Natural Deduction and Elementary Logic Textbooks by Jeff Pelletier and added several new links as suggested by Charles Stewart and falcon.]