The Pilot’s Handbook of Aeronautical Knowledge (2021 edition) is available or free from the FAA’s Website in PDF format.
If you have an iPad and would like the PDF available in iBooks, I have a page that lays out how t do so.
I’m the kind of guy that needs a hard copy in his hand. One that I can highlight, make bookmarks, and scribble in. If you are like me, Amazon sells the hard copy.