Harold Lamb
 

His histories and biographies are some of the most enjoyable I've ever read, but Lamb was also a writer of adventure fiction. His Cossack and Viking stories put him in the realm of Howard, or more correctly, Howard took the realm of Lamb's work and used it to help build a world of his own. Though Lamb's tales were not sword and sorcery, he proved that the sword could be more than enough to enthrall a reader and show what an exciting and adventurous world this could be.
 

 
Fiction
 

A Garden to the Eastward
The Curved Saber
Durandal
The Grand Cham
The House of the Falcon
Kirdy
Marching Sands
The Mighty Manslayer
Nur Mahal
Omar Khayyam
Riders of the Steppes
The Sea of the Ravens
The Skull of Shirzad Mir
Swords from the Desert
Swords from the East
Swords from the Sea
Swords from the West
Swords of the Steppes
The Three Palladins
Warriors of the Steppes
White Falcon
Wolf of the Steppes

Non-fiction
 

Alexander of Macedon: The Journey to World's End
Babur the Tiger: First of the Great Moguls
Charlemagne
Chief of the Cossacks
The City and the Tsar
Constantinople: Birth of an Empire
The Crusades
Cyrus the Great
The Earth Shakers
The Flame of Islam
Genghis Khan: The Emperor of All Men
Genghis Khan and the Mongol Horde
Hannibal: One Man Against Rome
Iron Men and Saints
The March of Muscovy
The March of the Barbarians
New Found World: How North America Was Discovered and Explored
Suleiman the Magnificent
Tamerlane
Theodora and the Emperor