I can't quite recall which episodes of Columbo I first saw, though I have distinct memories of watching some kind of rerun of the first two episodes of season 8, Columbo Goes to the Guillotine and Murder, Smoke and Shadows on television. Though I think I already knew Columbo at that point, so that means I had already seen episodes before those...
The Grassy Knoll is a novel that often feels very much like a Columbo story like you'd see on television, but at the same time, it often feels very much unlike a classic Columbo story. Some elements, I will let slide because this book was published in 1993, so after the (relatively) newer series of Columbo which sometimes do have a different vibe compared to the original series. A bit more sex, Paul Drury apparently liking to walk around naked, not exactly what I expect from classic Columbo, but I could imagine things like that in the series from season 8 on. And the fact that the narration actually refers to Mrs. Columbo, confirming her to be absolutely alive and all is also something later Columbo seasons did, having third parties confirm her existence, though I must admit I always loved the ambiguity regarding her existence of the earlier seasons. But on the whole, we have an inverted mystery story set in the flashy television world of Los Angeles, we have murderers who think they are thousand steps ahead of Columbo and make fun of him only to find that the man is slowly but surely learning the truth by asking a lot of questions and finally, it all comes falling down. In that sense, The Grassy Knoll is of course what you'd expect of a Columbo novel.
But one thing that does make this feel very much unlike any other Columbo stories is the focus on the JFK assassination. Apparently, the other Columbo original novels by Harrington also tackled real world crimes, but it's just something I didn't really like about this book, as it is definitely more just 'fluff' or a thing only Paul Drury was on about, the actual murder becomes a major theme of the book when Columbo starts suspecting Drury's obssession with the case is what led to his murder, so some parts of the book have Columbo actually looking into the JFK assassination and learn the details about that death and theories regarding the "true" shooter. It is weird seeing Columbo investigating a real world crime, and while he doesn't come up with some history-altering theory about this murder, it was still something that felt out of place to me, even though it is apparently Harrington's hook for this series of books.
The mystery plot itself is also slightly different from what you'd expect of a Columbo. Yes, it is an inverted mystery, with the murderers having created a false alibi for themselves for the murder, but this element isn't even the most important part of the story: the false alibi is torn apart rather easily, and when you come to the end, you'll realize there's not really a "big" satisfying moment where Columbo laid a clever trap, or where the murderers made a truly "oh, in hindsight I should've seen that coming" mistake (their biggest "mistake" was just having a rather simple plan...). The murder itself, and the way Columbo solves whodunnit are Columbo-esque in form, but in terms of feeling as satisfying as the best of Columbo episodes, like the gotcha moments in episodes like Suitable for Framing or A Case of Immunity, you won't find that here. It doesn't help that Tim and Alice aren't really interesting opponents either. What the mystery is mostly about, is the reason why Tim and Alice killed Paul Drury. We soon learn that Tim and Alice are actually in contact with a third person in regards to this murder, and most of the mystery for the reader is figuring out why Tim and Alice decided to kill the star of their show. This quest into the motive brings Columbo even outside Los Angeles for a short period, and ultimately links back to the JFK assassination in some way, but as I mentioned above, I didn't really like the real-world crime connections here, so it kinda fell flat for me. I think the idea behind the motive itself is interesting, just that it doesn't really belong in Columbo.
As a 1993 book, the book is interesting in the way it was modern for the time when it comes to the usage of computers, but it's really dated when you read it now, which is quite funny. We're not just talking about the police doing 'zoom and enhance' on pictures and having to explain what a virus is, but also Columbo being impressed by computers with dozens of megabytes of storage space or computer back-ups on hundreds of floppy disks.
I wouldn't say The Grassy Knoll feels completely unlike a Columbo story, for that is not true. It has all those trademark moments and lines you should expect of a Columbo tie-in novel. But the murder mystery itself is not particularly memorable and when the book goes deeper into the matter of motive, it does feel like it's doing something you normally wouldn't expect of the series, and your mileage may vary on how much you like that. Tone-wise, the book is also a bit closer to the last few seasons of Columbo, which I'll admit are not my favorite seasons, so that plays a role too in how I feel about the book. But still, it was perfectly fine for a book I found in the free library!