The Secret Country Series by Pamela Dean

What happens when your fantasy world turns real...? Each vacation for the past nine years, cousins Patrick, Ruth, Ellen, Ted, and Laura have played a game they call the “Secret”—an invented, scripted world full of witches, unicorns, a magic ring, court intrigue, and the Dragon King. In the Secret, they can imagine anything into reality, and shape destiny. Then the unbelievable happens: by trick or by chance, they actually find themselves in the Secret Country, their made-up identities now real... moreWhat happens when your fantasy world turns real...? Each vacation for the past nine years, cousins Patrick, Ruth, Ellen, Ted, and Laura have played a game they call the “Secret”—an invented, scripted world full of witches, unicorns, a magic ring, court intrigue, and the Dragon King. In the Secret, they can imagine anything into reality, and shape destiny. Then the unbelievable happens: by trick or by chance, they actually find themselves in the Secret Country, their made-up identities now real. The five have arrived at the start of their games, with the Country on the edge of war. What was once exciting and wonderful now looms threateningly before them, and no one is sure how to stop it… or if they will ever get back home.   "An intricate sparkling web of intrigue and magic. One of me very favorites.”—Patricia C. Wrede, author of Dealing with Dragons less

The five cousins are still trapped in the Secret Country, and must play their parts. When the King is poisoned, Ted-Prince Edward-must take the throne, even though he has no idea how to rule a country, battle magic, or inspire followers.  Soon enough he will have to do all three because the Country is on the verge of war with the treacherous Dragon King.

The Third and Final book of the Secret Country Trilogy!   Three things have the power to destroy the Secret Country: the Border Magic, the Crystal of Earth, and the whim of the dragon. The cousins Ted, Laura, Ruth, Patrick, and Ellen have faced the first two; now, summoned back to the Secret Country, they must face the third... moreThe Third and Final book of the Secret Country Trilogy!   Three things have the power to destroy the Secret Country: the Border Magic, the Crystal of Earth, and the whim of the dragon. The cousins Ted, Laura, Ruth, Patrick, and Ellen have faced the first two; now, summoned back to the Secret Country, they must face the third. The Country's most trusted counselors now know that the five are impostors, somehow thrust into the roles of royalty, but no one knows who has been playing with their destinies. The truth lies with only Chryse, the unicorn, and Belaparthalion, the dragon. But getting to them, and speaking with them, is more complex and dangerous than it seems….   “Pamela Dean’s Secret Country books are required reading for anyone who loves fantasy. Get them!”—Will Shetterly, author of Dogland less