The most excellent folks at PhotoPills have created another most excellent free ebook, Sunset Photography: The Definitive Guide. The ebook is 144 pages long and totally free for the downloading. Though the assumption is that you are using their most excellent phone app, PhotoPills, there is much to learn in the guide even if you are not.
Download here: http://static.photopills.com/ebooks/photopills-sunset-en.pdf