Popular free-to-play kids virtual world Poptropica has grown its book line with tomorrow’s launch of four new book titles.

Two new titles, Skullduggery Island by Adrianne Ambrose (paperback and eBook for kids ages five and up) and Astro-Knights Island by Tracey West (paperback and eBook for kids ages six to eight), expand the interactive storylines from some of the site’s most popular islands. A sneak peek of Skullduggery Island can be found at Funbrain.com.

The two other books, Pencil Warrior by Poptropica (paperback for kids ages six and up) and Poptropica Mad Libs by Roger Price and Leonard Stern (paperback for ages eight and up), feature storylines and activities. Pencil Warrior is a full-color doodling book while Poptropica Mad Libs includes 21 stories based on games and missions from the virtual world.

More information about the books including the existing Poptropica: The Official Guide can be found at www.poptropica.com/books/.

Poptropica is the creation of Jeff Kinney, the author of the bestselling “Diary of a Wimpy Kid” series. Since launching in 2007, the site now boasts more than 250 million visits by kids worldwide and was named as one of TIME magazine’s top 50 websites of 2011.

In addition, more than 60 million friend connections have been made globally in the site’s safe social network offering, Poptropica Friends.