It's complicated. The unpublished work is certainly in the public domain insofar as copyright is concerned. If the work was edited for publication, those changes might be copyright. The final book is a compendium of materials including the autobiography, front materials, and so forth. The ancillary materials may have a different copyright than the body of the book. If there is only one copy of the unpublished work, even if copyright does not apply, other property rights may. Nothing is ever easy about copyrights.

What can be copyrighted is frequently confused. In particular (Section 102 USC17):

102(a). Copyright protection subsists, in accordance with this title, in original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device. Works of authorship include ...

102(b). In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.

On top of this expression/idea dichotomy there is a large body of case law which further explains what is meant.