There are 3 books in the Last Herald-Mage series. The newest book is Magic's Price which came out in 1990.

1
Magic's Pawn

Book cover of Magic's Pawn
Groundbreaking epic fantasy series in Mercedes Lackey’s Valdemar universe • Lambda-Award winning novels with heartfelt high adventure and magic  
 
Though Vanyel has been born with near-legendary abilities to work both Herald and Mage magic, he wasn’t no part in such things. Nor does he seek a warrior’s path, wishing instead…
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2
Magic's Promise

Book cover of Magic's Promise
Groundbreaking epic fantasy series in Mercedes Lackey’s Valdemar universe • Lambda-Award winning novels with heartfelt high adventure and magic  
 
Wild magic is taking its toll on the land. Many Heralds and Herald-Mages have died fighting to preserve the peace. Even Vanyel, the most powerful of the Herald-Mages is almost at…

3
Magic's Price

Book cover of Magic's Price
Groundbreaking epic fantasy series in Mercedes Lackey’s Valdemar universe • Lambda-Award winning novels with heartfelt high adventure and magic  
 
Valdemar—the once-peaceful kingdom protected by the magic of its Herald-Mages—is now besieged on all fronts. The king lies near death, the neighboring land of Karse wages a relentless war against Valdemar,…