These are some of the courses that I found :
-1. (https://www.coursera.org/specializations/unity-xr)
-2. (https://www.coursera.org/learn/augmented-reality) [This course is really good. But don't expect to learn about ARCore or ArKit.]
-3.(https://www.coursera.org/learn/ar) This course is provided by Google, but has very basic information and can be completed in 3-4 hours.
Text
1. Unity 2018 Augmented Reality Projects (You could use the O'Reilly 'Free Trial' to see the content of the book. https://www.oreilly.com/library/view/unity-2018-augmented/9781788838764/)
-You could also join Unity Connect group
(https://connect.unity.com/i/7dbcaa8c-5f27-436f-bfc4-f6ef22749ddc)
-Please feel to add more!