There are many online books about Rust. Here is popular 5:

1. The Book
https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html - you start here
2. Rust by example
https://doc.rust-lang.org/rust-by-example/std/result/question_mark.html - the same concepts as in The Book but oriented more on practical examples
3. Cookbook
https://rust-lang-nursery.github.io/rust-cookbook/file/read-write.html - about using crates in real-life problems, solutions, best practices
4. Rustonomicon
https://doc.rust-lang.org/nomicon/send-and-sync.html - when you are ready for something spicy
5. Learning Rust
https://learning-rust.github.io/docs/combinators/ - and back to origin

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References and Borrowing - The Rust Programming Language

The following books were harder to find but nevertheless their content is golden

6. The Embedded Rust Book
https://docs.rust-embedded.org/book/peripherals/singletons.html - Rust for solving tasks with microcontrollers
7. Easy Rust
https://fongyoong.github.io/easy_rust/Chapter_48.html - if you find The Book confusing (as me) then try this

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Singletons - The Embedded Rust Book