There are 5 books in the Unicorns of Balinor series. The newest book is Night of the Shifter's Moon which came out in 2000.

1
The Road to Balinor

Book cover of The Road to Balinor
As Arianna recovers from a bad accident and loss of memory, she slowly recalls things about her past and learns that her “horse” Chase can speak thoughts to her, as can her dog. When Chase is threatened, Arianna tries to run away with Chase and Lincoln – only to find…
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2
Valley of Fear

Valley of Fear
A fantastical tale for middle readers presents the dramatic tale of Sunchaser and Princess Arianna and their desperate search for the Royal Diadem and Princess Arianna's parents. Original.

4
The Secrets of the Scepter

The Secrets of the Scepter
Beginning a pilgrimage to learn the powers of the recently acquired Royal Scepter, Princess Arianna hopes to reach the celestial unicorns before the evil Shifter attacks again

5
Night of the Shifter's Moon

Night of the Shifter's Moon
At the next new moon, or shifter's moon, Princess Arianna will lead the battle to reclaim the royal throne and defeat Entia