Topos Theory
by Peter T. Johnstone · Peter T. Johnstone
Research-level monograph by Peter Johnstone, a principal reference for the subject, developing toposes systematically from their categorical foundations. Readers with solid category theory gain a rigorous command of elementary and Grothendieck toposes, geometric morphisms, and internal structure.
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More resources on Topoi
Wolfram MathWorld
MathWorld is an online mathematics encyclopedia from Wolfram Research offering detailed, browsable articles on topics across the math spectrum, including algebra, geometry, calculus, and number theory. Each entry includes definitions, theorems, formulas, diagrams, worked examples, and links to further reading.
nLab: Topos
Reference article from the nLab, a research wiki maintained by working category theorists, defining Grothendieck and elementary toposes and surveying their logical and geometric interpretations. Readers get precise definitions, standard examples, and pointers to the primary literature and related entries.
nLab
Collaborative wiki for mathematics, physics and philosophy written from a category-theoretic viewpoint, with cross-linked entries on categories, functors, adjunctions, topos theory, higher category theory and homotopy theory. Readers can look up precise definitions, examples and references for advanced structural mathematics.