The official Avid help manuals are really dry and can hard to ingest (at least for me). While informative, they read like VCR repair manuals rather than consuming a well worded lecture. The Davinci Resolve manual on the other hand, reads like a polite conversation and flows very well.
The Avid Handbook by Steve Bayes has always been a personal favourite go to resource for learning Media Composer. It's dated now, but worth buying the $5 used copy on Amazon. https://www.amazon.ca/Avid-Handbook-Techniques-Strategies-Information-ebook/dp/B00979YGEQ/ref=sr_1_2?keywords=avid+steve+bayes&qid=1578426006&sr=8-2
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Avid is packed with a lot of legacy, and many things work exactly the same way now as they did 20 years ago when I first learned the software. The newer features you can pickup by scrolling through the press, and Avid's own website, for new features added to major releases.
The Avid Blog is really helpful, with quick gif animations walking through new tools: http://www.avidblogs.com/tag/media-composer/
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Michael Phillips' blog 24p is an excellent resource for odd n sods and troubleshooting specific Avid things, especially on the workflow side of things. http://24p.com/wordpress/
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As for everything else, I have found that if you ask very specific questions you will usually be greeted with a polite enough answer on this thread. liftgammagain.com is another great resource, along with creativecow.net