I think you look too much into that stuff to be honest (regarding the books) as we've discussed before in the thread and already disagreed about. We've had this conversation before. This series doesn't take it's self serious, and has a lot of satirical nuance as I've pointed out before. It is supposed to be cartoony and goofy while dealing with very real life situations and cases. I believe in one of the commentaries, Marlowe indicated that they researched all of their cases that they explore and most of them are based on real life cases.
Is there a case that the show has jumped the shark and become a parody of it's self in the past two seasons? Perhaps. I'll grant, especially since we have focused more and more on the relationship between Castle and Beckett. I'll even go so far as to agree with you that the quality of this season has been off considerably compared to the previous three seasons. Aside from last night's episode and perhaps a couple of others I really don't have a top five yet. Perhaps this will change when we enter May. The second half of last season for example with the Captain Montgomery arc. I'm really shocked that we're 21 episodes into season four and not really have dealt with the JB case yet or had a JB centric case. I'm guessing they're saving that plot for May sweeps next month. I know we both are interested in different aspects of the series, you the mystery plot, me the shipper aspect but they're also very much intertwined and I guess we're going to have to disagree about that. I just don't think "Castle" was meant to have an all out realistic approach. There are nuances and other things that I pointed out that contradict that IMO, this is why it is so FUN compared to other more "serious" and reality routed procedural shows.