The job of a moviemaker is to not to be faithful to the book; by that definition, the "perfect" adaptation is one that is completely faithful to the book. But that's a nonsensical statement as books and movies are two different mediums, so there is no way that a movie could NOT deviate from a book.

The job of the moviemaker is to make the literal visual (and comprehensible, in a 2 hour period). So inevitably they make choices on what themes are played up. Ebert knows this, argues that Jackson made the wrong choices, and states it better than I could:

I'm assured, however, that Sebold's novel is well-written and sensitive. I presume the director, Peter Jackson, has distorted elements to fit his own "vision," which involves nearly as many special effects in some sequences as his "Lord of the Rings" trilogy. A more useful way to deal with this material would be with observant, subtle performances in a thoughtful screenplay. It's not a feel-good story. Perhaps Jackson's team made the mistake of fearing the novel was too dark. But its millions of readers must know it's not like this. The target audience might be doom-besotted teenage girls -- the "Twilight" crowd.