Question: How many books are there in the Pretty Little Liars series?

Pretty Little Liars (book series)

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Pretty Little Liars is a series of young adult novels by Sara Shepard. Beginning with 2006's initial novel of the same name, the series follows the lives of four girls– Spencer Hastings, Hanna Marin, Aria Montgomery and Emily Fields.
The novels have appeared on The New York Times Best Seller list.[1][2][3] A television series adaptation loosely based on the novels debuted on June 8, 2010, on ABC Family. The franchise has been licensed by Amazon.com's Kindle Worlds range of non-canon ebooks.[4]
Overview
The series is loosely divided into four arcs, chronicling the introduction and reveal of each "A". Set in the suburbs of Philadelphia, the series follows the lives of four teenage girls nicknamed the Pretty little Liars or simply the Liars, whose clique falls apart after the disappearance of their Queen bee leader, "Alison DiLaurentis". A year after her disappearance, the girls begin receiving threatening messages from an anonymous character named "A," who threatens to expose their secrets, including long-hidden ones they thought only Alison knew. Shortly after the messages begin, Alison's body is discovered buried in her yard. The books progress with the four girls trying to figure out the identity of "A" and facing many dangerous obstacles and twists as they do so.
Characters
The series has many characters, most of them recurring. The main characters are:
Spencer Hastings: Spencer is a headstrong, intelligent, overachieving girl who comes from the wealthy Hastings family.
 Hanna Marin: Hanna is the bubbly, strong, and fierce "it-girl" of the group following Alison's disappearance. She suffers from an eating disorder, about which she received taunts from Alison.
Aria Montgomery: Aria is a quirky and artsy girl who, at the beginning of the books, is trying to find herself.
Emily Fields: Emily is considered the jock of the group, being a physical, loyal girl who struggles with her sexuality.
Alison DiLaurentis: Alison is the former "Queen Bee" of her clique, consisting of Spencer, Hanna, Aria and Emily. She had supposedly been killed at the beginning of the series however is revealed to have been alive and also their tormentor "A" as she was mistaken for her twin sister Courtney who pretended to be like her and was sent to The Preserve at Addison-Stevens, which she believed was the Liars' fault and has been plotting revenge ever since.
Her identical twin sister Courtney DiLaurentis is not crazy but she debuted in<i data-parsoid='{"dsr":[5129,5139,2,2]}'>Wanted after the TV show began in 2010. In the prequel, Courtney-as-Ali started disappearing in the last chapter and getting almost killed numerous times, until the end of the series (Vicious) when Ali-as-Courtney came out and started becoming friends with her twin "Ali" again after rehab with Iris and Tabitha, two lookalikes of Ali, all in the penultimate chapter.
In the book, their older brother's name is Jason DiLaurentis and their half-sisters are Melissa and Spencer Hastings.
Courtney DiLaurentis: Alison is really Courtney in the books. She used to be sweet, until she started pretending to be her twin, where her personality morphed into her. She is the third child to Jessica and Kenneth and was sent to The Preserve at Addison-Stevens after a physical argument with Alison who she eventually turned into and befriended the Liars up until the night of her infamous disappearance when she was killed and buried by the real Alison DiLaurentis and her accomplice Nick Maxwell.



A: A is the main antagonist of the series, being, for the most part, an anonymous character who relentlessly torments many characters, in particular the Liars. Mona Vanderwaal, Alison DiLaurentis and Nick Maxwell are discovered to be A. Mona became A due to Alison and the Liars being responsible for The Jenna Thing, which burned her too. However, she is killed when Spencer pushes her off a cliff face and her neck is caught between rocks. However, a new "A" rises, this time even more determined to hurt the Liars. This A is revealed to be Alison herself who wanted revenge on the Liars for ruining her life and Nick, who is revealed to be her accomplice.
Novels
First Arc

Second Arc

Third Arc

Fourth Arc

Companion novels