Feedback by people not directly working on a document conversion may be helpful, as these people usually see problems not obvious to the converter himself. Also, the earlier there's feedback, the earlier any fundamental problems can be fixed, and the less damage is done by a wrong decision during the specification of the conversion goals.
Such helpful feedback may come from collegues either working on book conversions, too, or not, from any local supervisor or even from the project leader. The persons who have to be asked for in any case are the author of the book and the publisher, as it is their document that is being converted, and the work is being done for them.
As mentioned above, feedback is welcome at every point during
document conversion, but when the conversion is done, the publisher
and the author should have a look again to give their final ok
before the document is delivered.