Rowena tells Charlie that The Book of The Damned is written by a witch named Agnes that was kind of a nutter, which is a reference to the book Good Omens: The Nice and Accurate Prophecies of Agnes Nutter, Witch by Neil Gaiman and Terry Pratchett.
Crowley (Mark Sheppard) is punishing a demon by throwing darts at him. He then sticks a handful of darts in, to which the demon replies, "Nice grouping." This is a reference to Young Frankenstein (1974) when Inspector Kemp (Kenneth Mars) cheats at darts by running up to the board and sticking in a handful of darts when Dr. Frankenstein (Gene Wilder) has his back turned. He turns and sees the darts and responds, "Nice grouping."
The brothers first met Charlie in season 7 episode 20: "The Girl With The Dungeons and Dragons Tattoo".