Here are a few:
Politics and Statecraft in the Kingdom of Greece 1833-1843 by John A. Petropoulos
Otho I, King of Greece: A Biography by Leonard Bower and Gordon Bolitho
A King's Private Letters by Constantine, King of Greece
Helen, Queen Mother of Rumania by Arthur Gould Lee
No Ordinary Crown: A Biography of King Paul of the Hellenes by Stelio Hourmouzios
A Measure of Understanding by Queen Frederica of the Hellenes
Frederica, Queen of the Hellenes: Mission of a Modern Queen by Lilika S. Papanicolaou
H. M. Konstantine XIII, King of the Hellenes by G. Nicholas Tantzos
Anne-Marie, Dronning Uden Rige by Sara Blaedel
La familia de la reina Sofía by Ricardo Mateos Sáinz De Medrano
Irene de Grecia, La Princesa Rebelde by Eva Celada
My Fifty Years by Prince Nicholas of Greece
Memoirs by Prince Christopher of Greece
A Romanov Diary by Grand Duchess George of Russia
H.R.H. Princess Marina, Duchess of Kent by James Wentworth Day
Mémoires insolites by Prince Michael of Greece
For a King’s Love by Queen Alexandra of Yugoslavia
Marie Bonaparte by Celia Bertin
Alice, Princess Andrew of Greece by Hugo Vickers
The Royal House of Greece by Arthur Gould Lee
Kings of the Hellenes by John Van der Kiste
The Inheritors of Alexander the Great by G. Nicholas Tantzos
The Royal Hellenic Dynasty by Prince Michael of Greece, Mrs Helen Helmis-Markesinis, and Arturo E. Beeche
The Royal House of Greece by Alan Palmer and Prince Michael of Greece
Greece and the English: British Diplomacy and the Kings of Greece by Panagiotis Dimitrakis