I also want to say Scenes from an Italian Restaurant by Billy Joel, but that wasn't the 60s. It's upbeat, has a wedding, the guy's name is Eddie (girl is Brenda). Nothing about writing or paperbacks, though.

As an aside, that was my favorite song when I was nine years old. :)

Brandy (You're A Fine Girl) - Looking Glass

obscure suggestion:
Ma Belle Amie - Tee Set

I love the song "Brandy". But that isn't it. Nor is it The Ballad of John and Yoko.

It might be "Dear Eloise" by the Hollies. It's upbeat, could be mistaken for the Beatles, and references both writing and a girl named Eloise.

Dear Eloise, I am writing to say
A number of funny things I heard today.
I heard that he's left you and run off to sea.
Could be the best thing that's happened to me.

Writing a letter to make you feel better.
Sorry to hear that he left you that way.
I could've told ya, he was much older,
So much older than you.

You rushed in blindly. He treated you kindly,
Until he found out what he could get from you.
You should've departed before he got started.
Now you've been burned, just like I said you would.

Please read my letter closely. It's beneficial to you.
Must read in between the lines, a message you will see.
If you use your mind, you'll find
I want you back with me.

Writing a letter to make you feel better.
Sorry to hear that he left you that way.
I could've told ya, he was much older,
So much older than you.

Dear Eloise, I am writing to say
A number of funny things I heard today.
I heard that he's left you and run off to sea.
Could be the best thing that's happened to me.