Since the Solana codebase is in Rust, it's important to understand the language. Here are some learning references:

• The Book
• Rust By Example
• Start here if you want to learn just enough to read and understand the code, or only have ~30 minutes
• Programming Rust is a well-written book, full of useful examples beyond the basic examples of The Book
• Coming from C++/C#/C/Java and want to learn Rust through building an application? This one is for you.
• Rust Language Cheat Sheet
• Rust Memory Container Cheat Sheet
• Learn Rust With Entirely Too Many Linked Lists
• The Rustonomicon
• The Rust Performance Book
• Common Rust Lifetime Misconceptions
• The Little Book of Rust Macros
• Rust Design Patterns
• Idiomatic Rust
• Rust API Guidelines

For even more, including podcasts/videos/etc, check out: Where to go to learn Rust in 2021.