First off, I want it to be known that this is not a book for younger readers. Much of the subject matter involves topics that are just too complicated for some younger readers. Now that doesn't mean that some advanced younger readers won't be able to read this maturely, but offhand I'd say that this book is best for readers who are 16 & above. Any parents that have kids that are interested in this series should read the books first & see if it's right for their kids. (It's a great series, so there is the danger of the parent swiping their kid's copies before they can read them!) 
 The first book in the series introduces the characters of Spencer, Aria, Emily, & Hanna. These girls used to be the best of friends... until their friend Alison mysteriously dissapeared one night at a sleepover. After that disastrous night they all went their seperate ways for the next few years, only to once again reunite in high school. Each girl has secrets that they don't want anyone to know. Spencer's intense rivalry with her sister Melissa has her trying to beat her in every category- even in the boyfriend category. Aria is trying to forget everything in her past even as she seems doomed to repeat her father's mistakes. Emily is trying to be the daughter her parents want... but how will they react when they discover that she's a little *too* friendly with the girl next door? And the formerly chubby Hanna tries just a little too hard to be thin and popular- she's willing to do anything to stay on top, even to the point of hurting herself. Now even with this powder keg of secrets, someone is watching and trying to push these girls beyond their limits. This mysterious person not only knows all of their sordid little secrets, but they also seem to share the first initial of a certain someone who went missing so many years ago... 
 I really didn't know what to expect from this series. At best, I was expecting a "Gossip Girls" clone. At worst I was expecting another "Ooh, we're rich, pretty, & biotchy. Love us!" type of book that really has no true reading value. I was incredibly shocked by the quality in this book. Not only are all the characters incredibly interesting, but the author manages to get one of the gold rings in literature: she makes her characters sympathetic & interesting even though each one is completely unlikable! That's right, you can't help but feel sorry for these girls even as you curl your lip at how despicable each one of them are. 
 Now some people have complained about the sexual situations in the series, but I have to say that the sex isn't over the top or glamorized. In fact, the sex is often portrayed in a negative light. Rather than have these girls hooking up with someone for love, they hook up (or try to hook up) for power or to be liked. The result is that the girls look pathetic & weak, which is the author's intent. Most teens over 16 will be able to detect the difference between someone hooking up for pathetic reasons & those doing it for lust, but younger readers may not be able to understand the difference. That's why I'm again stressing that parents will want to preview this series for their younger readers & discuss the themes in the book. Not only will you appear that much cooler for knowing the series, but it's a good way to discuss the more delicate topics such as sex, alcohol, & self-esteem without seeming preachy. (Plus the books are just really darn good!)