Can't agree more with the above two posts.
The FAA's Pilot's Handbook of Aeronautical Knowledge is an incredible resource. It's available for free on the FAA site in PDF or you can buy the paper version. It basically includes all you need for PPL level and a lot of what you do for ATPL. There are a few minor differences between EASA and the FAA, but it's nothing major.
https://www.faa.gov/regulations_poli...aviation/phak/
Another thing you could do is take a look at some maths and physics preparation books. A good understanding of the maths and physics involved in aviation will give you an excellent start.
The Padpilot one is free:
https://books.apple.com/gb/book/math...ts/id592113679
CAPT (Phil Croucher) has a free one available on their site too:
http://www.captonline.com/samplenotes.pdf