The 2004 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.