A book on Category/Topos Theory could serve as a mathematical foundation. From it you can create various logic systems (including intuitionist logic) as well as locally-true set theories. Two inexpensive books that cover this are
Topoi: The Categorial Analysis of Logic by Richard Goldblatt
Toposes and Local Set Theories: An Introduction by John Bell
Both are published by Dover, a publisher that produces rigorous, high-quality texts for a very affordable price. I highly recommend Topoi, but unless you've had an extensive training in rigorous thinking it may come off as pointlessly abstract and useless. However, if you approach it with no expectations you will gain a deeply profound view of mathematics.
Good luck.