Rust and its community value open-source a great deal, thus there is lots of excellent resources available online. Almost all the packages on crates.io have an online documentation on Docs.rs, which covers the API and general information. That documentation is written in the form of comments in the code. More elaborate Rust projects have specific book-like online documentation generated by mdBook to share concepts, ideas and information that is not related to one specific package. I would like to recommend some of these mdBooks, as well as a print book.
The best place to start of is the official Rust book The Embedded Rust Book by Nichols and Klabnik . The book covers the concepts of the programming language with many examples. It is available as online documentation or as print.
The Embedded Rust Book by Blandy and Orendorff  is another great resource, especially for those who already know C++, as it makes many comparisons between the languages.
To start out with Rust on microcontrollers I recommend reading the The Embedded Rust Book by Embedded Rust Workgroup . It covers the additional information needed to compile and run Rust code on bare-metal devices.