Begin with the "A Tour of Mathematica" section for a high-level overview of capabilities.
The Mathematica Book, Fifth Edition by Stephen Wolfram is a 1,488-page comprehensive guide and the definitive reference for Mathematica Version 5. Although released in 2003, it remains a valuable resource because it details the core principles of the Wolfram Language that still power modern versions of the software. Core Sections of the Book The Mathematica Book, Fifth Edition
This section serves as a readable tutorial for basic operations. Begin with the "A Tour of Mathematica" section
Functional programming, pattern matching, transformation rules, and how Mathematica represents and evaluates expressions. Part 3: Advanced Mathematics in Mathematica The Mathematica Book, Fifth Edition