nLab: Sheaf
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Wiki reference entry giving the general definition of a sheaf on a site, with the gluing condition, sheafification, sheaf toposes and many worked examples. Best used to check formal statements and trace references after a first textbook treatment.
More resources on Sheaves
Stacks Project
A collaborative, open-source reference on algebraic geometry run from Columbia University, with thousands of pages building from commutative algebra to schemes and algebraic stacks. Every result has a stable tag, so readers can look up precise statements and complete proofs.
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.
Math 216: Foundations of Algebraic Geometry (The Rising Sea)
Ravi Vakil's Stanford course site hosting free drafts of The Rising Sea, a graduate text on Grothendieck-style algebraic geometry: sheaves, schemes, morphisms, dimension, smoothness, quasicoherent sheaves and cohomology, with extensive exercises. Readers learn to work fluently with schemes and the modern foundations of the field.
Sheaves in Geometry and Logic
Mac Lane and Moerdijk's standard introduction to topos theory, starting from sheaves on topological spaces and building to Grothendieck topologies, geometric morphisms and the internal logic of a topos. Readers gain a working command of sheaves as both geometric and logical objects.
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.