My favorite vanilla tickling in books was in the Xanth fantasy series for teens, by Piers Anthony. Multiple fun mentions in various books, several seriously hot. Here's one example I just found online. Not sure if it's from Castle Roogna or Centaur Isle:
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Today [Irene] was dressed in a green blouse and skirt that accentuated her figure and wore green slippers that enhanced her fine legs and feet. In short, she had prepared well for this encounter and had no intention of letting him write his essay in peace.
She took a deep breath, inflating herself against him. "I'll scream," she breathed in his ear, taunting him.
But Dor knew how to handle her. "I'll tickle," he breathed back.
"That's not fair!" For she could not scream realistically while giggling, and she was hyper-ticklish, perhaps because she thought it was fashionable for young ladies to be so. She had heard somewhere that ticklishness made girls more appealing.
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I read that over 35 years ago. Already well aware for years that I had a tickle fetish. The idea that girls would want to be more ticklish to be more appealing was incredibly hot.
There are great - brief and vanilla, but with clear sexual tension - tickles between main characters in multiple Xanth books. Others noticed too. I just found this statement in a review of Vale of the Vole:
"I started to realize that the tickle giggles were turning into seductive sexual molesting between the protagonist Esk and the antagonist Metria."
There are also mentions by Robert Heinlein in several (most?) of his sci-fi books. Only extremely brief and passing. But he has a male protagonist tickle a female love interest enough that I wondered if the author is one of us.