Recently I purchased a copy of 'Card Design Handbook' by Paper Crafts. To save on the shipping (which is more expensive than the book itself), I bought the digital version. I actually have a few card making books, but most of them focused on techniques. The 'Card Design Handbook' is all about the principles of card design eg. the composition, balance, colours etc with lots of card examples. I'm new to cardmaking, so I thought it might be a good investment. And I really love what I get!
Here's one card that I made, while trying to apply the 'quiet space' and 'sweet spot' principles to the card design.