Honest, Humble, Intelligent
As our narrator, Ivan sets the tone for this book. And since he's a pretty humble and honest guy (er, gorilla), the tone of the book is, too. Though we understand that he's pretty darn special (his Free Ruby campaign is impressive, to say the least), part of his charm is that he doesn't lay claim to this about himself. Ivan's just being Ivan, as far as he's concerned, and if we recognize him as extraordinary, well, that's our problem.
To this end, Ivan tells his story in an up-front manner, sharing the world as he sees it honestly—but since he's so smart, this gets pretty intellectual at times. For example:
I wish humans could understand me the way I understand them. (the loneliest gorilla in the world.99)
It's a simple statement, yet it manages to demonstrate everything about the tone in this book. Here, Ivan is straightforward and forthcoming about his feelings (a.k.a. honest), he shares his wish without hating on human intelligence or identifying himself as superior to those who can't understand him (think: humble), and he demonstrates a whole heckofa lot of insight into both himself and those around him (so he really brings the intelligence). And this, Shmoopers, is how Ivan rolls on every page of the book.
Seriously. Close your eyes, let the cover fall open, and point your finger anywhere on the page. Chances are more than decent that when you open your eyes and see where your finger's landed, it'll be a moment where Ivan is recounting his life with honesty, humility, and major smarts.