Some time ago I did a round-up of the discrepancies between Zimmerman's SPD statements, and the FDLE summary of Osterman's statement to them.
It's not the statements of Osterman to the FDLE that are the issue. The FDLE report is a summary of his interview and not admissible (it would only come up if Osterman testified differently at trial and they called the agent who interviewed Osterman to impeach him, not Zimmerman, with the prior inconsistent statement.
But what Osterman writes in his book and says publicly about what George told him could potentially be used to impeach George at trial if George testifies differently from what Osterman says in the book or on TV that he told him. All the state has to do is ask George on cross-examination if he ever said X. If George denies saying X, they call Osterman to the stand and ask if George ever told him X. If Osterman says yes, it's impeachment evidence.
So the statements that could hurt Zimmerman are those Osterman directly attributes to Zimmerman in his book or in media interviews. I think those would be the statements to compare with Zimmerman's statements to police.
Treehouse has the supposed excerpt of the chapter 1
and will be publishing a new one each day.