The Prisoner of Zenda
by Anthony Hope.
This is my favourite book and it has all the right ingredients. It is a historical adventure that includes the handsome hero, the beautiful, sad princess, the not-very-nice king, the king's wicked brother, the baddie who is not really that bad (he's actually nicer than the king and his brother), and unrequited love. It also has the added ingredient of an identity swap.
Rudolf Rassendyll is an Englishman on a fishing holiday in the mid-European Kingdom of Ruritania. He is noticed by the king's henchmen who realise that he is the spitting image of their royal master, (also, coincidentally, named Rudolf), who is due to be crowned the next day. Rudolf is invited to dine with the king in his nearby lodge, but during the meal, the king drinks some drugged wine (thoughtfully provided by his wicked brother, Black Michael) which means he is out for the count and unable to attend his own coronation.
Rudolf is persuaded to impersonate the king for the day because, if he does not turn up for his coronation, his brother, Prince Michael, will take the crown instead and this would be disastrous for the kingdom. During the coronation, Rudolf meets Princess Flavia, who is soon to marry the king, and falls in love with her. She cannot understand why the king is a much nicer person than she thought he was, and she falls for him. (Actually she falls for Rudolf Rassendyll).
Meanwhile, Michael has found out where the real king is and has had him removed to a place of safety, (namely a dungeon), by his henchman, Rupert of Hentzau (the baddie).
As you can see, this is getting complicated. I won't tell you any more of the story - read it for yourself. It is a rollicking good yarn, with sword fights and all that sort of stuff. The author keeps the pace up throughout and there's an exciting ending. One person dies, another has a narrow escape, another puts duty before love, another...