Don't think anyone has mentioned the wonderful "My Son, the Double Agent" by Ted Mark.
The book is a tongue-in-cheek spy type novel. Think of the sort of spoof films that grew up around the era of the early Bond movies and you’ll get the idea.
The hero is a guy called Steve Victor, who’s an agent for the O.R.G.Y organization. On the cover of the first copy I had he just happens to look like a dead ringer for the 1960's era Sean Connery.
Right at the beginning of the story, Steve finds himself captured by a beautiful female agent from a rival organization. Having thought he was about to jump into bed with her, Steve’s already naked, when suddenly he gets overpowered. He’s then forced to bend over double and each of his wrists gets tied to the corresponding ankle. His legs are spread wide apart and his calves get lashed to a pair of bedposts so he can neither squat or stand up. He is now staring at his captor who he sees upside down as he looks through between his legs. Then the fun starts!
The girl starts asking him questions. She wants to know who he is, who he’s working for, details of his mission…all that sort of stuff. When he fails to answer, she produces a long goose feather, approaches him from behind, and starts tickling his balls.
It is at this point that we discover that Steve Victor is extremely ticklish! In fact, he bursts out laughing immediately.
In what is a very, very good torture sequence, the girl continues to tickle Steve Victor’s balls with the goose feather, whilst demanding that he answers her questions. He is very quickly laughing so hard and helplessly that at one point he actually thinks he’s going to be tickled to death. He really is absolutely howling with helpless laughter throughout.
Not long after, Victor just can’t take any more and he tells the girl he’ll talk, if only she’ll stop tickling him.
Shortly after this, Victor escapes. However, he is soon re-captured and brought to a barn for a second interrogation. When he enters, he sees a different woman sitting stroking the same goose feather. He is so ticklish that he has to stop himself from giggling, just from anticipation! Unfortunately, no further tickling ensues in this case.
The book is written in the first person, so the whole narrative is given from Steve Victor’s point of view. There is lots of detailed description of Victor’s laughing. I hope I’m not breaching copyright, but here’s a small example –
Domino delicately dipped the tip of the feather between my thighs from behind so that the spheres of my manhood swayed ever so slightly.
“Ha-ha-ha-ho-ho-ho-hee-hee-hee-ha-ho-ha-hee!” Thus it started.
“Why did you kill our man in Manila?”
“I didn’t kill anyone in Manila!” I tried to tell her.
She ignored my protest. The feather wiggled again.
“Hee-hee-hee-hee-ho-ho-ho-ho-ho-ha-ha-ha-ha-hee-ha-ho-ha-hee!”
It is well worth tracking down.