This is a classic little book on spurring creativity. This is also the author of A Kick in the Seat of the Pants
An illustrated deck of 64 creative thinking strategies. You might also be interested in Innovative Whack Pack
Learn to think like one of the most creative minds of all time! They also publish a workbook for this called The How to Think Like Leonardo da Vinci Workbook: Your Personal Companion to How to Think Like Leonardo da Vinci
This is a physical object you interact with to create ideas. I'm not entirely sure how you use it, but it is such a different approach to creativity I can't help but believe it would help you come up with ideas.