Too Much Magic - Jane Tesh

Coronation Day looms for Mel Worthington, surprise heir to the throne of Eldenfair, a fairy tale land located on a vast alternate world called Andrea on the Other Side of his human home.  Mel is also the grandson of Sorena, Eldenfair’s most powerful sorceress.  Sorena has mysteriously disappeared, so Mel must depend upon her familiar, the snake Sleek, to guide him as he learns how to control his own unexpected magic.

Warned by the court wizard, Zorill, of a possible takeover attempt by a rival wizard, Mel and his Captain of the Guard, Riley Evensong, are searching for answers when they fall through doorways of light, landing in various Andrean lands, all threatened by the Cavern-born, evil beings set on conquest.

Mel has three days to get back to Eldenfair or forfeit the crown.  More importantly, can he, Riley, and the rabbit Timmon, Official Talking Animal Sidekick, solve the mystery of the doorways and survive the Cavern-born invasion?