This idea was suggested on another thread. Feel free to add on as I'm just going to name a few to start off. Of course, some authors may post under assumed names and not want their identity disclosed. If that's the case, please respect it. I'm going to start with those who openly discuss their books.
Ilana Miller (Ilana)
The Four Graces
Grand Dukes (contributor)
Reports from America: William Howard Russell and the Civil War
contributor to Royalty Digest Quarterly and co-editor and contributor to European Royal History Journal
Royal Mob (fiction under the name Theresa Sherman)
Royal Gatherings I
Royal Gatherings II
Arturo Beeche (Eurohistory)
The Coburgs of Europe
Dear Ellen
Royal Gatherings I
Royal Gatherings II
founder and contributor to European Royal History Journal
Grand Duchesses
The Grand Dukes
The Other Grand Dukes
King Michael I of Romania
The Gotha - Still a Continental Royal Family, Volume I (Volume 1)
Royal Hellenic Dynasty (with Prince Michael of Greece)
My Fifty Years (added to the memoirs of Prince Nicholas of Greece)
Empress Zita of Austria
Grand Duchess Josephine Charlotte of Luxembourg
Prince Louis Ferdinand of Prussia and Grand Duchess Kira Kirillovna of Russia
Queen Juliana and Prince Bernhard of the Netherlands
The Dowager Empress Marie Feodorovna of Russia
King Frederik IX and Queen Ingrid of Denmark
Russia and Europe - Dynastic Ties (with Galina Korneva and Tatiana Cheboksarova)
APAPA
The Nassaus of Luxembourg
Grand Duchess Maria Pavlovna
Marlene Eilers (Marlene)
Queen Victoria's Descendants
The Gleichens: the Unknown Royal Cousins (lengthy article available for purchase)
The Grand Dukes
The Other Grand Dukes
Greg King (Greg King)
The Grand Dukes
Gilded Prism
Resurrection of the Romanovs
Fate of the Romanovs
The Court of the Last Tsar: Pomp, Power and Pageantry in the Reign of Nicholas II
The Duchess Of Windsor: The Uncommon Life of Wallis Simpson
Sharon Tate and the Manson Murders
The Last Empress. The Life And Times Of Alexandra Feodorovna, Tsarina Of Russia
Mad King a Biography of Ludwig II of Bavaria
Twilight of Splendor: The Court of Queen Victoria During Her Diamond Jubilee Year
The Other Grand Dukes
The man who Killed Rasputin: Felix Yusupov and the murder which helped bring down the Russian Empire
A Season of Splendor: the Court of Mrs Astor in Gilded Age New York.
Penny Wilson (Penny Wilson)
Gilded Prism
Resurrection of the Romanovs
Fate of the Romanovs
The Grand Dukes
The Other Grand Dukes
Helen Azar (Helen Azar)
The Diary of Olga Romanov
RUSSIA'S LAST ROMANOVS: In Their Own Words. (with Margarita Nelipa)
MARIA and ANASTASIA: The Youngest Romanov Grand Duchesses In Their Own Words: Letters, Diaries, Postcards.
Journal of a Russian Grand Duchess: Complete Annotated 1913 Diary of Olga Romanov
Janet Ashton (Janet Ashton)
The Grand Dukes
German Woman
The Other Grand Dukes
Sarah Miller (Sarushka)
The Lost Crown
Miss Spitfire
Lisa Davidson (LisaDavidson)
The Grand Dukes
Helen RappaportMagnificent Obsession
The Last Days of the Romanovs: Tragedy at Ekaterinburg
Conspirator: Lenin in Exile
Queen Victoria: A Biographical Companion
No Place for Ladies: The Untold Story of Women in the Crimean War
Joseph Stalin: A Biographical Companion (Biographical Companions)
The Romanov Sisters: The Lost Lives of the Daughters of Nicholas and Alexandra (also called The Four Sisters in other editions)
Eric Lowe (Eric Lowe)
Royalty in Photographs
Royal Images
Ann LyonA Constitutional History of the United Kingdom
Richard Cullen (Richard Cullen)
Rasputin: The Role of Britain's Secret Service in His Torture and Murder
Nicholas 'Nick' Nicholson (NickNicholson)
Object of Virtue
Little Girl in a Red Dress with Cat and Dog
Margarita Nelipa (belochka)
The Murder of Grigorii Rasputin
Alexander III His Life and Reign
RUSSIA'S LAST ROMANOVS: In Their Own Words. (with Helen Azar)
Petra H. KleinpenningThe Correspondence of the Empress Alexandra of Russia with Ernst Ludwig and Eleonore, Grand Duke and Duchess of Hesse. 1878-1916.
Douglas Smith (Prospector)
Former People: The Final Days of the Russian Aristocracy
The Pearl: A True Tale of Forbidden Love in Catherine the Great's Russia
Working the Rough Stone: Freemasonry and Society in Eighteenth-Century Russia
Love and Conquest: Personal Correspondence of Catherine the Great and Prince Grigory Potemkin
Eva and Daniel McDonaldFanny Lear: Love and Scandal in Tsarist Russia
Delin ColonRasputin and The Jews
David McIntoshEmpress Zita of Austria
Grand Duchess Josephine Charlotte of Luxembourg
Prince Louis Ferdinand of Prussia and Grand Duchess Kira Kirillovna of Russia
Queen Juliana and Prince Bernhard of the Netherlands
The Dowager Empress Marie Feodorovna of Russia
King Frederik IX and Queen Ingrid of Denmark
Virginia Rounding (VR)
Nicky and Alix
Catherine the Great
Grandes Horizantales
Elena Maria VidalTrianon: A Novel of Royal France
Madame Royale
The Night's Dark Shade: A Novel of the Cathars
Other than Marlene's article, which is available for separate purchase through her blog as well as being published in Art's magazine, I'm not going to list individual articles. However, in addition to many of the authors listed above, others have written numerous published and online articles:
Magazine & Article contributorsHelen Azar
Margarita Nelipa
Griffith Henninger--publishing his first article in the upcoming Royalty Digest Quarterly based on the work he started here long ago on Alix's war relief work!

Also, I know I have missed contributors to such works as Grand Dukes and Grand Duchesses. I will attempt to edit them in. I just don't have the books in front of me to doublecheck.