The Greatest Traitor, The Life of Sir Roger Mortimer, Ruler of England 1327-1330, by Ian Mortimer. I'm a big fan of Ian Mortimer, having read his Time Traveler's Guide to Medieval England: A Handbook for Visitors to the 14th Century several years ago. He writes about some obscure history in a really accessible and conversational way, which I love.
In short, the story of Roger Mortimer is a little interlude in the history of the monarchy that doesn't often get the attention it deserves. Basically, he was a nobleman based out of Wales, born in 1287. His life was pretty normal and uneventful until around 1322 when he revolted against Edward II.
So Edward II. Poor Edward II. What can we say about Edward II?
He was sandwiched in between two great warrior kings, Edward I and Edward III. He was the father of Edward III? He was well educated, and intelligent, and also had very bad judgment. He may or may not have had sexual relations with his dear friend Piers Gaveston. Whatever their relationship (ie sexual or not) he loved Piers to the exclusion of others, and he risked much of his kingdom to keep Piers happy, rewarding him with land and titles that the nobles didn't think he deserved.
(Actually, I've just found a really great blog that started from a person wanting to salvage Edward II's reputation, so I'm going to have to do some reading on that, to see if my mind is changed.)
So the court was political and there were lots of factions. To say the least.
And Mortimer comes along, and he's all, "yeah, this king sucks, I'm going to join the movement to rebel, and we're gonna kick some ass."
Except he got caught and was imprisoned in the Tower of London. From which very few people ever escape. In fact, Roger Mortimer was one of the very few people who have escaped. At the time of his escape, he was the first. He escaped by drugging his guards thanks to the help of a sympathetic guard, escaped through the kitchens and across the river, and eventually to Dover and then France.
Where he wound up taking as his mistress... wait for it... the Queen of England, Isabella, who was also sick of her husband and laying low in France. Her young son, Edward III would eventually join the couple, and they would plot to invade England.
Poor Edward II (that's really all I can say about him now - Poor Edward II) is captured and imprisoned. There are rumors that he was never killed, but somehow managed to live out his life in obscurity in Italy. But he was most likely killed. There are other rumors that he was sodomized by a hot poker, a token to his supposed relationship with Gaveston. No one really knows for sure.
What we do know is that for several years, until Edward III came of age and got sick of being a puppet for Isabella and Mortimer, Roger Mortimer essentially ruled England. He wasn't a king, he had no royal blood, but he was the ruler.
Eventually Edward III plotted his death, with a bunch of his equally-fed-up friends, and in the middle of the night they went through a tunnel in Nottingham Castle that went straight to Isabella's rooms, the door of which was conveniently left open by a sympathetic guard (sympathetic guards were very useful to have around) and Edward III came in with his buddies, swords drawn, and caused a bit of a scene.
Mortimer was subsequently taken to Tyburn, despite Isabella's pleas, and hanged like a traitor. They didn't take him down for several days so everyone could get a good look.
Other than when Oliver Cromwell ruled as Protectorate, I can't think of any other times since 1066 (other than Regency's) when someone without royal blood has so clearly ruled England as a king himself. That in itself makes him an interesting guy to study.