BJ is reading the first Letter encountered in the game.
Letters can be found throughout Return to Castle Wolfenstein, written between characters in the game and often providing insight into the current state of affairs.
For a complete listing of clipboards and letters from Return to Castle Wolfenstein, see List of Clipboards and Letters. For intel from the prequel missions only found in console ports of the game, see List of Clipboards and Letters (Cursed Sands). The font is written in Comic sans.