Topos Theory
Peter T. Johnstone
Topos Theory - A book resource
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More resources on Topoi
nLab: Topos
Comprehensive wiki entry with references
Toposes, Triples and Theories
Free online book on topoi
ncatlab.org
nLab is a collaborative, research-oriented wiki offering in-depth articles on category theory, topology, topos theory, higher categories, logic, and related topics, with definitions, explanations, and references for advanced mathematics.
mathworld.wolfram.com
MathWorld is an extensive online mathematics encyclopedia by Wolfram that offers concise, authoritative explanations, definitions, formulas, and diagrams for a wide range of mathematical topics. It includes detailed entries on higher‑level concepts such as topoi in category theory, with cross‑references and suggested resources for further study.
ocw.mit.edu
MIT OpenCourseWare provides free, openly accessible MIT course materials—lecture notes, assignments, exams, and video lectures—across a wide range of subjects for self-paced study and teaching.