The Assassin's Curse: Blackthorn Key, Book 3

Like the other Blackthorn Key books, The Assassin's Curse has a raft of historical detail and arcane facts (the Knights Templar play a big role). Christopher shares much of his knowledge of chemistry along the way, as well as his talent for solving codes. Since the story's set at the court of Louis XIV of France, there are a lot of French words and phrases, most helpfully translated. Also Latin and a famous classical painting.

Harrowing adventures often involve false identities, art thefts, and other normally criminal behavior in the king's service, but teens Christopher, Tom, and Sally, along with faithful pigeon Bridget, rise to the challenge with talent, courage, and sometimes deadly force. A character gives a mortally wounded villain poison to spare her a long, agonizing death; in the past, Templars are slaughtered and burned at the stake. The wisdom of Christopher's late Master Benedict, both potion-brewing and life lessons, comes in handy.  There's much appreciation for everybody's skills and talents, as well as lifesaving teamwork.

There's a growing attraction between childhood friends Christopher and Sally, now about 13, but they're mostly occupied with other things. Frequent references to lovers and mistresses at court.