This book has only two problems that keep it from being a masterpiece:
1. Mr. Denning later wrote The Leader's Guide to Storytelling which covers the same material much more thoroughly and helpfully; and
2. The fable is built around a mythical company of squirrels that provides nut burying services but is having problems. I had a hard time getting my mind around that metaphor. By contrast when Dr. Spencer Johnson teaches me about mice being in trouble when their cheese is moved, I don't have the same problem with the metaphor.
I recommend that you mainly use this book as a humorous side trip into leadership story telling. If you don't fell like you need such a humorous journey, you can certainly skip this book.
If you do skip Squirrel Inc., I urge you to read Mr. Denning's two "serious" books on the subject in this order: Start with The Springboard and then move on to The Leader's Guide to Storytelling. I've written reviews of both of those books recently if you want to check them out. Both are superb and are must reads for any leader who wants to be effective.
If you have to choose between reading Leading Change and The Springboard, choose The Springboard.