I am writing a historical novel that concentrates on the space of time between the death of King Edward VII dying and the coronation of King George V. I am planning on it being focused on George's relationships with others and his worries about becoming king. The thing I'm worried about is that I am playing with History here, and I don't know how much freedom I really have. I can find several sources of his activities, but I don't know if changing the events would be wise, and if I can, by how much?
Well, it's not mean to be a biography, is it. Some poetic license has to be okay. Take the movie Braveheart, for example. Riddle with inaccuracies. But that did not stop it being very successful.
That's why it's called historical fiction. I have a new appreciation for just how hard it is to write historical fiction as I read someone else's work. Here's what I'm finding so far. There's a difference between a period piece of fiction and historical fiction. In the former you try to get the setting and character interactions to match with the period. In the latter you add in or focus more about known historical events. It's my understanding (not that I know anything) for example, you could write about a sailor on one of Columbus' ships on his first voyage. That would be historical fiction. Or you could write about a sailor on a ship in the late 1400s. That would be a period piece. In both cases you need to do a lot of research about the era, that's where you risk messing up. In historical fiction if you said so-and-so was the ship's captain and actually it was someone else, I don't think most readers would care. Who knows many names on those three ships except Columbus, and if they did the reader would probably just chalk it up to the fiction part of the genre. But if you said Columbus had respect for the people he found and the reader was familiar with Columbus' actual journals or Howard Zinn's more evidence based work on history, a reader like myself would balk.
Few historical records provide details of everything the happened around the significant characters and the significant events that make "history". So long as the landmark events and decisions are unchanged, and the characters behave in general compliance with what is generally known about them, it should be fine as a piece of historical fiction.
If you haven't read her already, check out Philippa Gregory's novels...she's a big name player in historical fiction involving the English monarchy.
I think this is one of the hardest genres to write in. If you're a history student already, then yay, this may be the genre for you. But there are so many details about a particular place, particular time period and particular people that you really have to get right. So saying that plenty of biographical pieces of fiction tend to get the details wrong either by design or error, and they go on to entrall audiences everywhere. As long as you're not calling it a biography as such you can get away with inserting suppositions about the character of the people involved, their motivations and even their personalities. People tend to enjoy these conjectures when placed in the context of fiction about persons of interest. It gives them something more than history does.
I think the best thing is that you study about real character of king Georg. I don't know, but may there are some books herein. Also you can know him through the historical events that he had a direct role in them. You even can know him (a bit) from his portraits ( his poses and behaviors)
There are many varieties of historical fiction (as opposed to period pieces, as @GingerCoffee rightly points out). You are definitely "playing with history" as you put it, but presumably not so much as to be writing what has become known as "alternative history", which posits a change in historical fact and then carries that to a logical conclusion (a good example is Philip Roth's The Plot Against America, which presumes that Charles Lindbergh became a "Stop Willkie" candidate in 1940 for the Isolationists, won the Republican nomination and defeated FDR in 1940, thereby keeping the US from entering World War II). What you are doing sounds much more in line with Michael Shaara's The Killer Angels or Herman Wouk's The Winds of War, although a key difference between the two is that the main focus of Killer Angels was a real person about whom the book filled in fictional details, while the main focus of Winds of War, Pug Henry, was a fictional character who interacted with historical persons who are presented in limited scope. Your statement of being focused on the future king's "relationships with others and his worries about becoming king" suggest you are going Shaara's route. That will require extensive research, if you're to get the details right. If there were any books written by those who were close to him, that would be best. Biographies would also help, although they may not provide a balanced presentation of the man. You can presume or invent aspects of his personalities, but they will need to be consistent with what is actually known about him, or else your work will not be credible. For example, the difficulties in Abraham Lincoln's relationship with his wife, Mary, as portrayed in Gore Vidal's Lincoln were well-documented in histories and journals. You may also want to create a fictional character through whom we can get to know the future king. All in all, it sounds like a fascinating project. Being in the editing phase of a historical novel, I can tell you that digging into the history and weaving the story is a very fulfilling exercise. I wish you the very best of success.