I'm gonna copy over my response from this topic (https://www.17thshard.com/forum/topic/90305-is-there-a-brandon-book-that-explains-the-shards/):
If you're up to date on all Cosmere books, then go ahead and read up on the Coppermind.
The book that is supposed to talk about the Shards, "Dragonsteel" hasn't been released. It's an upcoming project of Brandon's.
Among the published books, your best bet would be the aforementioned Arcanum Unbounded: a collection of short stories set across various planets in the Cosmere, they contain short paragraphs explaining the planet's history & magic. There's a short but succinct explanation in Mistborn: Secret History by Khriss, as others have already mentioned, that you can look forward to. There's also The Emperor's Soul, which I cannot recommend enough, it contains a good, simple explanation of the 3 Realms by Shai.
Beyond that, there are resources in the forums that you can use, other people have asked for a basic breakdown of the Cosmere before:
https://www.17thshard.com/forum/topic/90696-i-just-want-a-simple-yet-in-depth-explanation/
https://www.17thshard.com/forum/topic/90756-spoiler-free-cosmere-guide/)
Plus there's a beginner's guide pinned in the Cosmere discussion forum:
https://www.17thshard.com/forum/topic/88225-cosmere-101/
as well as over on the Columns & Features section of News:
https://www.17thshard.com/news/features/intro-to-the-cosmere-r570/
But more than anything, if you have any questions or doubts, just ask a question in the Cosmere Q&A forum