In conjunction with the opening book you also need games by strong players. You can download archived ICCF correspondence games that are in PGN format and are zipped for faster download. Games dating back to 2003 are available. Another source is the archived games from the now defunct IECG site covering its activity period from 1995 to 2010. An excellent source of otb games can be found at The Week in Chess.
After downloading the games and putting them in a DB folder you’ll need to weed out those played by all except the best players. You can decide on whatever rating you want for the cutoff point; I chose 2300. It was from these remaining games that I made my opening book except I upped the rating to 2400 for the opening book.
After you’re done you will have a database of games from which you can search out positions and see how the best CC and OTB masters played a position. What I usually do is select games that were won by whatever side I’m playing then run through them just to make sure that the win wasn’t the result of an opponent’s mistake later in the game. Once I’m reasonably sure I have some games that are well-played where my side won, I can follow them. Sooner or later it will be up to my opponent to find an improvement on the loser’s play. Sometimes they are successful, sometimes not, but the onus of improving play is on them. It makes for more successful results.