An A-Z of Visual Ideas: How to Solve Any Creative Brief (affiliate link) by John Ingledew, aims to help you solve visual creative deadlock. Organized in 26 sections following the letters of the alphabet, it introduces a number of concepts that you can use as the basis of your design. Examples: counter-intuition, eyes, juxtaposition, and zeitgeist.
It is written more with advertising or poster design in mind, but still it can help you broaden your creative mind with the concepts provided in the book, or by encouraging to think out of the (visual) box yourself.