The theoretical knowledge is notoriously demanding. Candidates must pass all 14 computer-based exams:
Since "ATPL EASA" is a broad domain covering regulations, operations, and flight training, I have selected an interesting paper that bridges the gap between and pilot psychology/decision-making . atpl easa
Easy Access Rules for Aircrew (Regulation (EU) No 1178/2011) The theoretical knowledge is notoriously demanding