'Pretty Little Liars': Author Sara Shepard to cameo

sara-shepard-pretty-little-.jpg "Pretty Little Liars" is welcoming series author Sara Shepard in a cameo role as a substitute teacher at Rosewood High School, ABC Family exclusively tells Zap2it.

Shepard will appear in the seventh episode of Season 1-- and ABC Family tells us she'll be having an unfriendly encounter with one of the girls ... but which one? You'll have to tune in.

Regardless of which girl it is, we bet the girls are psyched to have Sara on set. Star Lucy Hale tells us, "When I got involved in the show, I went and bought all of [the books]. I'm now on the seventh one ... I can't stop reading them. They're addictive."

Shepard is best known for her "Pretty Little Liars" YA series books, but she also published an adult novel, "The Visibles," in 2009, which got positive reviews from Publisher's Weekly. We can't wait to see her foray into the world of her books at Rosewood High School. Shepard's cameo appearance airs Tuesday, July 20 at 8 p.m. ET on ABC Family, which just might have a hit on its hands with "Pretty Little Liars" -- it debuted to good ratings in key demographics.

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Photo credit: Sara Shepard's official site