I tried some different books, but the only thing that ever worked was actually just doing it. I picked a project for myself and looked up different components to learn how to do it. I'm certainly no expert but this hands-on approach was the only thing that helped me learn at all. Mr. Excel and Google were my sources.
Being honest I haven't seen a book on Office scripts yet (I haven't particularly looked yet), hundreds on VBA but not Scripts. There are a fair few youtube video's (just do a search on "youtube office scripts").