Ned Ackerman appears to have written just the one book, Spirit Horse, though of course he might have written other books under different names. If anyone can help with any biographical information about the author, do please get in touch.
Finding the book:easy to find: still appears to be in print.
A young boy, Running Crane, longs to capture a fine horse of his own, and can’t help dreaming of owning the legendary stallion known as “the spirit horse”. He is separated from the rest of his group on a horse raid gone bad, and only he sees the spirit horse alone on the prairie. The boy tries to succeed in doing what the warriors have failed to do - capture and tame the majestic stallion.