Another good source is "Mastering the Galaxy S4" manual written by a CNET author. It's free if you own a Kindle (lending library) or only $3.99. I borrowed it on my Kindle and found it was pretty good if you don't mind reading pages and pages of manual-type material. Most of the stuff is pretty straightforward if you are pretty familiar with Android and smartphones but there are some gems in there.
Amazon.com: Mastering the Galaxy S4 eBook: Sharon Vaknin: Kindle Store