Genius Bonus: If you have a solid knowledge of history, literature, music, hypothetical physics, and computer programming then you're in for a treat.
Dirk's ramble about St Cedd "one of the duller Northumbrian saints. His brother Chad was even duller. Has a cathedral in Birmingham, if that gives you any idea." St Cedd's College is fictional, but its namesake was real, and was indeed the brother of Chad of Mercia, whom St Chad's Cathedral is named after.
Recycled Script: Douglas Adams, who liked to recycle plot elements, based the first Dirk Gently book on two of his Doctor Who scripts: "Shada" and "City of Death". Knowing this helps explain one of the stranger assumptions in the first book: just why Professor Chronotis happens to be a centuries-old man who lives in a time machine. (It's because he's a retired Time Lord.)
What Could Have Been: The Salmon of Doubt anthology features the beginning of a third book that was never written. And its style is so instantly familiar that it feels just like the other books. Then it stops. Just after Dirk has flown to America and encountered a truck carrying a rhinoceros to an unknown destination.
The TV series
Fridge Logic: Gordon's iPhone was sitting in Ruth's drawer for 16 years. Why didn't the battery run out?
Given that Jane/MAX's host body has been in a dark secret room in what amounts to a coma for 18 years, how is she even able to walk more than a few steps? She was using a cane to walk.
Ho Yay: Dirk and Richard have a terribly codependent and tsundere relationship. This may be also part of being a parody of Sherlock.
Tear Jerker: In episode 2 when Dirk finds out Jericho was the one who got him expelled, and only hired him as security as a Detective Patsy. The look on his face is heartbreaking.
