The Pilot’s Handbook of Aeronautical Knowledge is a free resource from the FAA. The FAA has the PHAK located at this link. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak
The handbook is broken up into many chapters on particular topics. Find the relevant chapter if you are struggling with an issue and want to learn more.
Aviation Attorney. FAA Certificated Commercial Pilot and Flight Instructor (CFI/CFII). Contributor at Forbes.com for Aerospace and Defense.