All three books of John & Carole E. Barrowman's HOLLOW EARTH trilogy. Book 1: HOLLOW EARTH. Book 2: THE BONE QUILL. Book 3: THE BOOK OF BEASTS.
Long ago, the Order of Era Mina bound all the beasts of myth and legend into the pages of a single tome. They called the prison they had created the Hollow Earth - a nightmare world built to keep our world safe. Over centuries, their Order grew strong: the men and women with the power to bind and animate the magic of this world learned to live in secret among us, watched over by their constant companions, the Guardians. Each Animarus was tasked with the protection of this world. Each Guardian was tasked with the protection of an Animarus. And in time the history of the Order was forgotten, their relics lost, and the Hollow Earth became nothing but a story.
Now, twelve-year-old twins Matt and Emily Calder are about to discover this world for themselves...

John Barrowman has worked in television, musical theatre, and film, and stars as Captain Jack Harkness in Torchwood and Doctor Who.