I prefer books because you can write with a pen into it. If you a good book to learn Arduino I advise you this one on Amazon: https://www.amazon.com/Arduino-Cookbook-Michael-Margolis/dp/1449313876/ref=sr_1_19?s=books-intl-de&ie=UTF8&qid=1507295426&sr=1-19&keywords=arduino+book&refinements=p_n_feature_three_browse-bin%3A4192709031
If you need some more projects or help you also can visit arduino.cc and go to the playground. There are some tutorials and projects with a building instructions.
If you also want to have a course than you should not take an online course, because the people there can't really see what you've builted, so they can't help you in some problems.
I learned Arduino from Margiolis' 1st edition "Arduino Cookbook". It has examples from blinking a LED to hosting a server in Arduino. I can only assume that the 2nd edition will be even better. Here's the book : https://www.amazon.com/Arduino-Cookbook-2nd-Michael-Margolis/dp/1449313876
I think that the Arduino Cookbook is a great book to use to learn Arduino.
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