The Xanth series by Piers Anthony has occasional good tickle references. A teenage girl in one of the early books in that series heard somewhere that it's attractive for girls to be ticklish, and suddenly decided she was hyper-ticklish. That fueled many fantasies... really wished certain girls I knew would do that! A few books later it was a recurring minor theme, as the main guy repeatedly got a cute girl to go along with things (not sexual, just whatever they were doing for the main plot) by threatening to tickle her.
Irresistible aside: tickling is the best way EVER for guys to control cute girls who are being stubborn, obnoxious, naughty, etc. It's brief, but wonderful. The rest of the time they have way too much power because they're hot.
Robert Heinlein books also occasionally mention tickling. At least one book (maybe Time Enough For Love) has several references, including one time a chapter ends with a hottie squealing, "Don't tickle!" and a strong implication that this was heading into the bedroom