Setting of Ranger's Apprentice, Book 4: Oakleaf Bearers

"Ranger's Apprentice: Oakleaf Bearers" by John Flanagan

This book happens around the medieval ages, around the year ~1000 due to the vikings, culture and weaponry. Two adventurers are trapped in a hostile country, and another group of two people sets out in rescue (these groups later merge when the find each other). One of the two adventurers is kidnapped by a mysterious horseman, who later turns out to be a Temujai, people to the far east who specialize in horsemanship. The second adventurer named Will (displayed on the cover) searches for his comrade. He finds a group of these Temujai, and starts firing arrows at them. Hearing the sounds of a skirmish, the original search party (With the another two adventurers named Halt and Horace, Halt being Will's mentor and Horace being Will's closest friend) comes to investigate. The battle is won with a prisoner captured. Following the path the Temujai took, they are ambushed by Skandians, who are the native people of the lands. Araulen, where the protagonists originate from, is at war with Skandia. Skandians are basically burly vikings. Hostile but interested with the prisoner, the Skandians and Araulans team up temporaraly to discover what's afoot. When it turns out that there is a horde of Temujai intent on capturing the whole continent, the two parties, with high-ranking officials in both of them, decide that their two nations must set aside their differences and fight the greater evil.