Asimov also made throw-away comic references to himself in a few of the “Black Widowers” stories, and used a caricature of himself as the science-writer guest in one of the stories (“No Man Pursueth”, IIRC).
The Late Great Ed McBain often referred to the movie The Birds in his 87th Precient mysteries. The screenplay to The Birds was written by Evan Hunter, McBain’s alter ego.
McBain also wrote in a non-87th mystery “Downtown” that the movie “Fuzz” was filmed in the protagonist’s hometown of Boston. “Fuzz” is an 87th precient mystery written by McBain.
Kinky Friedman became a bestselling author writing a series of mysteries about a country western singer turned dectective named–what else?–Kinky Friedman ;j
In George Pelecanos’ Soul Circus, the only book of his I’ve yet read (quite good, I’ll read more of him in future), one of the P.I. characters finds himself in a courtroom tracking someone down. There he very briefly encounters an unnamed court reporter (physically described in nothing approaching favorable terms) that is obviously Pelecanos – especially if you know what Pelecanos looks like.
–Cliffy
The Gor books mention the existence of the Gor books once or twice - in Assassin of Gor and Fighting Slave of Gor for sure, and in the latter book I believe one of the characters claims to have met John Norman.
Talking of breaking the fourth wall, I saw the other day the Wikipedia article on Loki from Marvel Comics, which mentioned that he is aware that he is a character in a comic. (So is the She-Hulk.) 
Mercedes Lackey does it, sort of. I’m rereading Bedlam’s Bard for the first time in years (I really need to go book shopping), and in the fourth chapter, the main character thinks, “Whoever came up with this should write a book. It’s better than half the fantasy schlock I’ve picked up lately–like telepathic horses, or ancient Aztec gods invading Dallas.” The telepathic horses and Aztec gods are references to two of Lackey’s other book series.
A chapter earlier, there’s also a random appearance by a “Misty” from Tulsa, OK, who shows up to fangirl the main character and then leaves. That seems like a pretty obvious self-insertion, though at least she doesn’t become a recurring character like the self-insertion she did in the Valdemar books. That was about the time I stopped reading those.
When Frederick Dannay and Manfred Lee started collaborating, they hit on the idea of making the name of their detective and the name of their pseudonym the same: Ellery Queen. After a few books Ellery became an author working for a living as the author of the Ellery Queen mystery novels. In several of the books, characters have read his books or even have a whole shelf of them and that’s why they call him in to solve the mystery.
When a secretary, Nikki Porter, was introduced in the Ellery Queen radio series, her character was later introduced into the print stories as well. (Many of what we now think of as basic parts of the Superman legend were also first introduced in the radio show and had to be brought into the comics.)
In what was supposed to have been their last book, The Finishing Stroke (Lee had developed enormous writer’s block), Ellery goes through a long flashback to 1929, when he had just published his first book, The Roman Hat Mystery (the real first book of the series) and attends a house party that his publisher gives in celebration. Of course murder ensues.
Later on, someone whose name I can’t think of and which will drive me crazy all day wrote a series of stories for Ellery Queen’s Mystery Magazine with titles like “the Ellery Queen Murder Case” and “The John Dickson Carr Murder Case” in which someone who is a fan of the books has to solve a case that uses the tricks and tropes that the writer was famous for.
But even this was anticipated all the way back in 1924, when Agatha Christie started writing a series of short stories (later collected in Partners in Crime) that were deliberate spoofs of the leading detective story writers of the day. Her heroes, Tommy and Tuppence Beresford, ran a detective agency and used the tricks and tropes of each writer, often by name, to solve the cases. The parodies were so nasty in spots that you have to wonder how Christie could turn around and write her own formulaic mysteries at the same time.
There are probably a zillion other examples of mystery writers referencing mystery writers. Chandler’s name gets thrown around an awful lot, for example.
I’m not sure the omniscient narrator is always supposed to be the author, but sure, it’s not uncommon. C. S. Lewis interjected himself into the Chronicles of Narnia, for instance–presumably to heighten the illusion that the stories had really happened and that he had been told about the Pevensie’s adventures by Lucy & her siblings. (It’s not clear to me how he found out about the events of The Last Battle without being dead himself.
My favorite one! (in no small way because of all the typography references)
In “Rosemary’s Baby” Rosemary mentions that other actors were working in road companies of {something or other} or Drat! The Cat. The latter musical was written by Ira Levin, who put a lot of real life details into the novel to make it seem real.
Hrm. If we take comic books into account Dave Sim in Cerebus eventually has (somewhere around issue 200 or so) the title character meet Dave himself. They converse in such a way that Sim convinces Cerebus that he’s the author and can do anything and that he’s the one responsible for everything bad and good that’s ever happened to Cerebus.
What’s special about that is Sim’s contention regarding that conversation. He states that his entire intent with the series was to get to the point where he and his creation communicate with each other. He says he had that idea in the late 70s when he started and worked for decades to get there. Woof, no wonder he went insane.
Oh, and another Heinlein sighting. Again in ‘The Number of the Beast’ in the final bit “L’envoi” he himself appears as a waiter and various other guises with names that are anagrams of his name.
Are you thinking of William Brittain’s “The Man Who Read _____” stories?
Forrest Gump met both Tom Hanks and Winston Groom in the godawful sequel Gump & Co. (which Groom admits without any embarassment [or need for] that he wrote only for the truckload of money delivered to his house for it).
Groom’s across the street neighbor Fannie Flagg has used a little girl named Patty Neal in some of her books (Fried Green Tomatoes and Coming Attractions among others) though her age is different in each one. Patricia Neal is Fannie Flagg’s birth name.
Not a book, but in the Steve Martin film The Jerk directed by Carl Reiner, it is director Carl Reiner who initiates the class action lawsuit about the Optigrab.
Tabby
That’s the one I came in to mention. What’s really great is that they often reference their own books very dismissively as trashy fiction. Although one character is reading The Ice Limit II which, while not existing, would make a helluva sequel, given the epilogue in the first book.
And Pendergast needs to be in a *good * movie adaptation.
I’ve never actually read the book, but doesn’t Philip Roth’s Operation Shylock involve Roth as a character?
It’s been a long time since I read it, but I believe that in an A. Bertram Chandler Grimes story, set in the Shakespeare Sector where reality is a bit tenuous, Grimes winds up on Chandler’s ship where Chandler is writing the Grimes stories.
In a similar vein, in one of the Tarzan books Tarzan goes to Hollywood, winds up on the set of a Tarzan movie, and is told he isn’t right for the part.
In addition to Stephen King’s self references, he and his wife Tabitha reference each others fictional worlds regularly. Tabithas town of Nodd’s Ridge (I think, and am too lazy to go confirm right now) was originated in “Mrs. Todd’s Shortcut” written by Stephen, and in her book “One On One” the basketball team plays against both Castle Rock and Derry. There’s a ton of cross references if you’re looking for them.
In “Lady Slings The Booze,” the protagonist Joe Quigley runs into Jake Stonebender of the Callahan series, who is a pretty close analogue of author Spider Robinson…
In a related vein, artists Brent Anderson and Alex Ross sometimes draw themselves (and writer Kurt Buseik) into the comic book Astro City, usually as small-time crooks who get busted by one of the local supers.
The TV show Bones is based on anthropologist Kathy Reichs’s novels about Temperence Brennan. In the show, Brennan writes novels about the anthropologist/detective Kathy Reichs.
And probably the most famous example in American Literature:
No mention of the brilliant movie Adaptation yet.