You can 

> ein Buch schreiben

that denotes the whole work, from start to finish.

And if you say

> ich habe ein Buch geschrieben

we know you have finished the book. 

And you can

> an einem Buch schreiben

which denotes being busy working on a book.

And if you say

> ich habe an einem Buch geschrieben

then we know you have been working on a book, but whether you have finished it or not is not said (most probably you have not, otherwise you'd been using the first phrase).

Also, compare the English *I wrote a  book* vs. *I was working on a Book*.

The same is obviously valid for any kind of book, including autobiographies.