None of which matters right now. No spoilers, but crucial to the plot of this episode is the series of detective novels created by characters in the show. It's the "Mrs. Melville" series and you can see several of the books in this shot here.

Got it?

On that shelf there are five copies of Mrs. Melville in London. On the top shelf three of them are the second, ninth and fourteenth books from the left-hand side.

It's not a great screenshot, I know, but you can see it clearly enough.

What about these?

They're also bad screenshots but it's definitely the same book. But this copy of Mrs. Melville in London is on the bookshelf in Jim Rockford's trailer in the Rockford Files episode "Rattlers' Class of '63"!

Interestingly, this entry in a fictional detective series is sharing one of Jim Rockford's shelves with an entry in a real-life detective series: Strike Out Where Not Applicable, one of Nicolas Freeling's Inspector Van der Valk novels.

But the real story is that this establishes that Lt. Columbo and Jim Rockford exist in the same television universe! It would be easy to imagine them running into each other at the chili joint that Columbo frequents, a little harder to imagine Columbo getting tacos on the pier like Rockford.