INTRO TO FORMAL LOGIC. A corrected version of the second edition of An Introduction to Formal Logic is freely available here. There are also links to answers to exercises and to various supplementary materials.
GÖDEL’S THEOREMS. Two books are freely available here, An Introduction to Gödel’s Theorems and the much shorter Gödel Without (Too Many) Tears, along with related materials.
CATEGORY THEORY Links to resources at an introductory/intermediate level, including lecture notes and freely available books. Also a link to my currently stalled but hopefully evolving Gentle Introduction to categories.
TEACH YOURSELF LOGIC You will find here the much-downloaded, and still developing, Study Guide for beginning mathematical logic, as well as other resources (including Book Notes reviewing many texts)
VARIA Links to various papers. Also links to snippets from over a hundred notes on logic for (more or less) beginners. Plus my old but still occasionally recommended advice on writing papers!
LaTeX FOR LOGICIANS Pages pointing to a wide range of resources of interest to logicians, philosophers and others using LaTeX.