The Pearls of Lutra (Redwall, Book 9)

When Ublaz Mad-Eyes sets out on a mission to steal Tears of All Oceans, a set of six spherical rose-pink pearls, from the Lutra otter clan (Lutra lutra being the Latin name for otters), Grath Longfletch must stand up to the evil pine marten while the Redwallers search for the missing treasures. Tansy, a young hedgehog living at Redwall Abbey, attempts to solve the riddle of the missing pearls while at the same time the evil marten, Mad Eyes, desires them for himself.