Depends on what aspect of React you're trying to learn. There's differences between trying to learn the core APIs, some of the flexible ways you can compose components together, or how React works internally.
The official React docs are the best place to start. After that, my React/Redux links list points to many good tutorials and articles to help learn React further. In particular, you might want to check out the React Component Patterns, React Component Composition, and React Architecture sections.
Agreed with that. There's also good community content here: reactjs.org/community/courses.html
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