Available on Amazon for about $50, What If? Writing Exercises for Fiction Writers (3rd Edition) is a great investment if you'd like a ton of writing exercises that focus on different important elements of creative writing including beginnings, character development, voice, dialogue, plot, etc. The authors also guide you with pointers with each exercise that are very helpful and often result in an "Ah-hah" moment if its an element where you were previously stuck. In fact, you'll probably see me refer to this book often in some of the exercises I share on this blog. If $50 is out of your price range, you can also purchase the 2nd edition for about $20 on eBay as I'm sure it's not too different.
Please feel free to offer any other book suggestions in the comments.