The Kings of Clonmel by John Flanagan

Will is at the annual Ranger Gathering but Halt is investigating mysterious happenings in the west. When he does finally return, it's with bad news. Hibernia is in turmoil. A religious cult calling themselves the Outsiders are sowing confusion and sedition, and five of the six Hibernian kingdoms have been undermined. Now the sixth, Clonmel, is in danger. Halt, Will and Horace set out to restore order. Can the secrets of Halt's past help them or will it jeopardize their mission?

Returning back to where he left off with at the end of The Siege at Macindaw, Flanagan give us another solid and exciting Ranger’s Apprentice story.