I know that I am really comfortable spending all my time looking for resources online rather than in music stores. When I look online I know that I will likely be spending no money at all, however sometimes spending a couple dollars for some physical material is worth it. Here is a book that I have found while surfing through Pinterest. I am really considering purchasing this book because it not only focuses on guitar but also on mandolin and ukulele, instruments I am also interested in playing (my future students might also benefit from this book as a resource.
This is one of the pages out of it that was displayed on Pinterest.