This book really helped me starting improvising
I was always so anxious that any tiny little modification might harm this cake recipe or that pancake variation, bcz - as we all know - "baking is like a science" (as opposed to cooking). But reading it, about a year ago, I figured out that there's a lot of place for creativity in my own baking!
I've started with Pound Cakes, then moved to Breads - all are based on really simple ratios and upon getting it, I feel much more free to improvise and modify.
As always, when you understand something, you're much more likely to succeed in it!