Featuring some of the best talents in all of comics, the 2007 series Immortal Iron Fist dives deep into the origins and legacy of the Iron Fist.
The 2014 series Iron Fist: The Living Weapon is a great place to start with Danny Rand. It's does a good job of introducing all the elements of the his story without being an origin story. It has a darker, more anime feel than most Marvel comics though, so it may not be for everyone.
Alternatively, the recent Power Man and Iron Fist is a much lighter series. It's full of jokes, has a very expressive art style, and it's great seeing Luke Cage and Danny Rand together. However, you won't learn very much about Iron Fist lore from this book.