I've been trying to read more Nancy Werlin lately for a number of reasons. I read The Rules of Survival last year and really liked it. Her early mysteries were also released (or rereleased) on audio with very appealing covers and copy descriptions, and she has a new book publishing this year. She's a perennial favorite among teens, as well.
She's also been the recipient of new covers for almost all of her books, including many interesting redesigns when the book goes from hard cover to paperback. Although her repitoire isn't as lengthy as, say, Richard Peck, she's been publishing for a while and the changes to the covers have certainly made even her older titles quite contemporary.
1994's Are You Alone on Purpose before and after. I think it's quite interesting that the gender of the main character on the cover is different.
1998's The Killer's Cousin before and after. I completely love the new cover. It has a very contemporary feel but it also feels timeless. The cover on the left feels like a 90s teen book design.
2000's Locked Inside doesn't work in the same way that The Killer's Cousin does for me. The new design, on the right, feels older than the cover on the left (the original). Very My So-Called Life.
2001's Black Mirror has two covers that work well, I think. I prefer the newer one on the right slightly more, as I think it's a littler clearer that the story focuses on a person of color. The one on the left gives a bit of the wilderness feel to it. I think the cover on the left might have more boy appeal to it, too.
2004's Double Helix with the audiobook cover on the left and the book's unchanged cover on the right. I love this book's cover: it conveys the science element quite clearly, and this is a title that stands out on a shelf.
2006's The Rules of Survival with the hardcover on the left and the paperback on the right. Talk about two different audiences. The left screams boy appeal while the right, in its purple, definitely has a better girl factor. This one works well both ways.
2008's Impossible with the hardcover on the left and the paperback on the right. I like both of them for different reasons. The one on the left feels fresh to me, while the one on the right feels girly. Both of those feelings work for me.
September 2010's Extraordinary has a really cool, girly cover to it. I will be interested to see how this one may get made over into paperback -- after I read it, of course.