A folder full of PGNs is fine, but a database unlocks superpowers. Tools like , ChessBase Reader (free), or Lichess studies allow you to:
She pulled out a battered notebook. “When I was young, I lost to a nine-year-old in a rook endgame. I wrote that game down. Then I rewrote the losing moves, added variations in brackets, comments in curly braces—Here I rushed. Should have activated my king first.”
For professionals: (expensive but gold standard).
: Despite being labeled "The Fundamentals" (Orange series), the material is notoriously difficult. Many masters recommend it for players rated 1400–1800 USCF