When I was in Junior High and High School, Edgar Rice Burroughs was one of my favorite authors. I read everything (including this book) I could. Of course, it didn't take long to realize ERB had many faults as a writer. Virtually all his heroes could be identified by the fact they had "black hair and steel gray eyes". You knew who were going to get married at the end of the novels because whenever the man and woman touched, they'd feel a spark of electricity at the touch. And, of course, Burroughs reused the same plots and plot elements over and over and over. But that was okay. Because you knew ERB wasn't trying to write great literature. He was writing manly man macho adventure stories. Some of which are referred to as Science Fiction, but the Science in these books was crap even at the time Burroughs wrote them. It's more accurate to say ERB wrote Fantasy Adventures.
Occasionally, Burroughs would venture into other genres, however. The Western genre was probably his second most commonly genre forays. Occasionally, he'd try his hand at History Adventure novels. But then there are those books like "The Mad King" that might be best categorized as a Medieval Adventure tale except it takes place in contemporary times. (Well, contemporary to Burroughs anyway.)
"The Mad King" takes place in the mythical country of Lutha. (Unlike other made up lands by ERB, this one doesn't have its own language. Part of the fun of Burroughs is learning the languages of the outrageous places he created.) Lutha's king had died some ten years prior to the start of the book and according to evil regeant, Peter of Blentz, the king's son, Leopold, then ten years old, went a bit crazy upon his father's death. This necessitated Peter locking the child away in an asylum. Peter of Blentz of course, enjoys ruling the kingdom of Lutha and and in order to insure he still rules Lutha, decides after ten years of living in an asylum, young King Leopold must die. (Why wait so long? Why didn't Peter kill Leopold 5, 6, 9 and a half years ago?) Just before Peter of Blentz can carry out his plan to kill the rightful king of Lutha, Leopold escapes. A description of the king is circulated for his recapture.
Enter the novel's hero, Barney Custer of Beatrice, Nebraska. (Unlike most Burroughs heroes, Custer has blondish red hair. But he has the "steel gray eyes".) Due to a bet he lost, Custer now sports a beard which along with the rest of his physical appearance, has him matching the vague description of King Leopold. Due to a joke Custer makes to Princess Emma Von der Tamm (of course, when Custer meets her, he doesn't know she's a princess. And of course, he falls in love with her instantly. And vice versa. These are usual ERB tropes.) and how he matches the vague description, Custer is mistaken for the king. (Custer compounds matters by saying he won't shave the beard as "a matter of honor" and it was either the beard or a wear a green bonnet [another possible penalty for losing the bet]. Why Custer doesn't explain he lost a bet and has to wear a beard for a year instead of saying it's a "matter of honor" is beyond me. At any rate, by phrasing it this way, Barney Custer comes across as a bit crazy to Emma.)
The book (divided into two parts) then revolves around Custer not only being mistaken for the king, but Barney acting as the king to save (twice) the kingdom of Lutha. Along the way, Custer has many adventures, rescues the princess, realizes he's of the same royal blood as King Leopold and eventually marries Emma Von der Tamm. Oh, and he repeatedly (in the first part of the book) tries to explain he's not the king. No one believes him, though you'd think Barney Custer would have a tell-tale American accent noticeable to the Germanic natives of Lutha. Not to mention that at one point in part one he provides evidence that convinces somebody he's not the king, evidence he's had with him all this time but never uses until this one time.
Edgar Rice Burroughs is a writer that as I grew more familiar with his works, I became more aware of his flaws. I still enjoyed reading his books, but I was aware of the flaws. Eventually, Burroughs became the writer I read when I wanted to park my brain in neutral and not think. Just enjoy the adventure. The problem with "The Mad King" is there are so many plot holes and lapses in logic, they jump out at you. It's hard to overlook them. It's not like reading a Barsoom book and knowing Mars isn't like that. These are flaws where characters have to act like idiots in order to advance the story. Because of this, while the adventure in "The Mad King" is fun, it's not as fun or interesting as a Tarzan or John Carter story.