Begin with the "A Tour of Mathematica" section for a high-level overview of capabilities.
Thousands of illustrative examples, including "Galleries" for formulas and graphics. The Mathematica Book, Fifth Edition
amazon.com/Mathematica-Book-Fifth-Stephen-Wolfram/dp/1579550223">bookstore or library near you, or The Mathematica Book, by Stephen Wolfram Begin with the "A Tour of Mathematica" section
This is the "deep dive" into how the language actually works. or The Mathematica Book