The policy aspect is changing so much so rapidly that the best source to keep up is a good news outlet. NYTimes does a good job. Just make sure you're not reading those "alternative facts."
The widely recommended book to learn about healthcare policy is Understanding Health Policy: A Clinical Approach, Seventh Edition by Thomas Bodenheimer and Kevin Grumbach (the 7th edition is slightly more up to date in current affairs, but the 6th edition should be fine).
Free online resources can be categorized as the following:
I second Lawper on the Commonwealth Fund. I LOVE their website, it's stock full of info not only on the USA health system, but also on a few other countries' health systems.