"Thank heaven, though, one of the few mistakes I haven't made is to talk about the unwritten book"

John Le Carre is describing the idea that he has made errors in his life, but he is happy that he has actually not made the error of discussing a book he has not written yet. This could be translated as a tip to himself to not make pledges he can not keep, or to not talk about things he has not yet accomplished. It might likewise be translated as a reminder to concentrate on today and to not get ahead of himself. By not discussing the unwritten book, he is advising himself to stay in the present and to focus on the jobs he has actually already accomplished. This quote might also be analyzed as a reminder to remain modest and to not boast about things he has actually not yet accomplished.