First published in 1953, Three Hearts and Three Lions is sort of the "older brother" to The Broken Sword. The two books share a medieval northern European setting and draw heavily from the mythologies of those lands, but the 1953 book is the one that more heavily influenced Dungeons & Dragons. The paladin class, the troll, the swanmay, and the struggle between Law and Chaos (from which Michael Moorcock drew inspiration as well), among others, all first appear here, making this one of the most important books for anyone interested in the pre-history of D&D. It's also worth noting that this book predates the appearance of The Lord of the Rings in 1954, so it's another bit of evidence for the fundamentally non-Tolkienian origins of the game.
Three Hearts and Three Lions is also useful in understanding D&D for other reasons. Firstly, its story of a Danish soldier during World War II being flung into a fantasy world is one more example of a "lost world" tale, one of many that lurks beneath the surface of the game. Secondly, the novel's equation of Law with the forces of civilization and the Church and Chaos with paganism and Faerie is clearly present in OD&D and is often overlooked in discussions of what alignment is and where it came from. Finally, because Three Hearts and Thre Lions draws on the romantic "Matter of France,"
it's a very clear case of a non-pulp fantasy influence on D&D (yes, I am aware of the irony of discussing it in this series of posts) and that alone makes it a worthy subject of study. That it's also a well-written and enjoyable book only adds to its significance.