There are 4 books in the Gateway Essentials series. The newest book is The Runestaff which came out in 1969.

1
The Jewel in the Skull

Book cover of The Jewel in the Skull
“Those who dare swear by the Runestaff must then benefit or suffer from the consequences of the fixed pattern of destiny that they set in motion. Several such oaths have been sworn in the history of the Runestaff’s existence...”—The High History of the Runestaff Dorian Hawkmoon, late the Duke of…
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3
The Sword of Dawn

The Sword of Dawn
Duke Dorian Hawkmoon's quest to destroy the Dark Empire of Granbretan leads him onto the path of a man who possess a rare ring that allows men to travel through time. Hawkmoon uses this ring to travel to a far future New Orleans, where he must battle the Pirate Lords…