Very unknown book, unfortunately. It is probably the best example of how lying affects you and other people. A girl named Gretchen is the main character. A new girl named Marybelle (I think) comes to her school, and starts telling lies. She tells a lie to Gretchen that she saw Gretchen's best friend, Susan, was stealing earrings at the mall. Gretchen half believes this, and when she goes to visit Susan, Susan (and her mom) treat her like she was the one who did something wrong.
With the help of her step-brother, Gretchen finds a way to figure out who is lying and what lie he/she told.
I read this book 3 years ago, it was really good. Hint: there are 2 of the word "liar" in the title. Does this mean there are two liars? Or a person that lied twice? Or something else?