Thing are not going so well at Fablehaven, the secret preserve for magical creatures.

Supposed friends have turned traitor, powerful artifacts have gone missing, the Society of the Evening Star is gaining strength.

And now the Knights of the Dawn must face their most fearsome challenge yet: dragons.

"I'm a huge fan of dragons," says Brandon Mull, author of the New York Times best-selling series of Fablehaven books, who is releasing Book Four of the series this week. "Dragons are the most iconic of all fantasy figures, but I've held off using them much until now."

Mull sees dragons as impressive, majestic creatures, but he thinks they all have differences. "No two dragons are alike in the Fablehaven universe. They are just like people. They have different abilities, different appearances, different thoughts. Some are nice; some are mean."

So what will it mean for the Knights to have to tangle with such creatures? Mull hopes it will mean an exciting, intense, fun read.

As the penultimate book in the series, "Secrets of the Dragon Sanctuary" (Shadow Mountain, $19.95), will resolve a few things, but it also must set up other things for the finale.

"I see the series building like a crescendo. The final book will be released in about a year. Right now, my wife is the only one who knows the whole story," says Mull.

What is certain is that thousands of readers all over the country will be caught up in the adventure. Book Three debuted at No. 3 on the New York Time best-selling series list and stayed on the list for a good while. Mull is hoping this one will do the same.

"The thing that I hear most often from people around the country is that they think the books are getting cooler, more exciting as they go."

And while Mull is honored by the reception of his work, he's also relieved. "This is an insecure kind of job," he jokes. "It's not easy to say to the world, 'I think this is neat, do you?' Or 'I made this whole thing up, will you buy it?' "

But so far, the answer has been a resounding yes. And not just for Fablehaven but for his other books, as well.

Fablehaven books are up to a 100,000-print run; the paperbacks are selling well; movie talks for both Fablehaven and his stand-alone "Candy Shop Wars" are "plodding forward." And in August, Mull will release his first picture book, a story called "Pingo," in which an imaginary friend becomes an imaginary enemy.

"It sounds kind of psychotic, but it's really a very sweet story about friendship," he says. It will be illustrated by Brandon Dorman, who has been the Fablehaven illustrator.