Here's a list of my favorite mature content themed books that I recommend to you.

Sequel of While You Were Gone and prequel of Book 2. Snippets from Descendants of The Sun after eps 16. A journey in Kang Mo Yeon's life as she finally takes the part of being with a soldier. There will be some extras featuring Yoo Si Jin.

Sequel of the Snippets. A journey in Kang Mo Yeon and Yoo Si Jin's life after tying the knot. Humor, romance, a bit drama, a bit action.