Functions, The Julia Language, 2024 - The official manual section providing a comprehensive overview of function definition, arguments, return values, and various syntactic forms in Julia.
Think Julia: How to Think Like a Computer Scientist, Ben Lauwens and Allen B. Downey, 2019 (O'Reilly Media) - An excellent introductory textbook that teaches programming fundamentals using Julia, including detailed explanations of function definition, parameters, and calling conventions.