Hatcher's Algebraic Topology (free PDF)
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Standard free textbook with detailed CW complex chapter
More resources on CW Complexes
nLab: CW-complex
Detailed reference with definitions and properties
mathworld.wolfram.com
MathWorld is Wolfram's comprehensive online mathematics encyclopedia, offering in-depth articles, definitions, formulas, and visual explanations across a wide range of topics. For CW complexes, you can find a dedicated article with formal definitions, examples, related topics, and references.
ocw.mit.edu
MIT OpenCourseWare offers free, openly accessible course materials from MIT across disciplines. You can find math and topology course content—lecture notes, problem sets, exams, and frequently video lectures—useful for studying CW complexes and related topics.
ncatlab.org
nCatLab (the nLab) is a collaborative online knowledge base for higher category theory and related areas of mathematics, focusing on formal concepts and their connections across topology and algebra. It offers wiki‑style entries, definitions, examples, and diagrams on topics such as CW complexes, homotopy theory, and other advanced ideas with cross-links and references.