Lesley is most known for her smash hit "It's My Party", recorded when she was just 16. But her most culturally significant hit was really "You Don't Own Me"--a proto-feminist anthem for women of all ages. I was also partial to "Maybe I Know" and, of course, "California Nights" (introduced on that "Batman" episode). You might not know that her producer, back in the day, was a young Quincy Jones or that her hit "Sunshine, Lollipops and Rainbows" was the very first hit song written by Marvin Hamlisch. And you might not know that Lesley and her brother Michael were nominated for an Oscar as co-writers of the song "Out Here On My Own", which appeared in the movie "Fame"; the film's title song, written by Michael, won the award.
As for Christmas music, Lesley didn't record any that I'm aware of during her peak hit-making years. She did record a handful of Christmas songs between 2000 and 2001, including the original "If Every Night Were Christmas Eve", but those remain officially unreleased (though I gather bootleg copies circulate through Lesley's fan club members...and, hey, if anybody wants to send me a copy... ;^) ). Her one commercially released holiday track (that I know of) is a 2004 duet with Clint Holmes (of "Playground In My Mind" fame), "Christmas In Las Vegas" from "Happy Holidays Las Vegas", a charity CD to benefit the Positively Kids Foundation.
You're gone too soon, Lesley. At least we'll always have "California Nights".