The Seventh Door is the third installment in The Children of the Bard series. It's an exciting book that you won't be able to put down.
The description on the back of the book is as follows;
An apocalypse approaches, and only Matt Bannister and his mother, Bonnie, can stop it. Forced to participate in the demon Tamiel's scheme, Matt, Bonnie, and Darcy (Matt's former foster sister) must travel to seven destinations. Behind each of the seven doors, they find a unique brand of evil that is designed to weaken Bonnie's soul-healing song and bring about God's wrath upon the Earth. In the meantime, Matt's twin sister, Lauren, searches for their father Billy Bannister, who has been captured by Tamiel's soldiers. Lauren is accompanied by Walter, Ashley, and the dragoness Roxil in a desperate attempt to rescue Billy before his captors can carry out their plan to kill him. Since Sapphira Adi has also been abducted Elam, Yereq, Makaidos, and Thigocia travel towards a stronghold designed to keep Sapphira's fiery powers under control. Along the way, a great disaster strikes that threatens their journey as well as Matt's and Lauren's hopes to complete their missions. By the time Matt and his companions reach the final destination, ancient mysteries are solved, dark secrets come to light, and they learn that opening the door will bring doom upon the planet. Yet, a great tragedy befalls the company, and the only remedy is to seek the power that lies behind The Seventh Door.