Appropriate recognition for contribution to a book I am a PhD student and my supervisor has asked me to make some significant contributions to a book which he is trying to publish. 

He has asked me to proofread the book (English is my native language but is not his) but has also asked if I could effectively rewrite some sections to improve the communication and structure of the book.

I am happy to do the work but would like to agree beforehand how my contribution should be recognised. As I will not be contributing any technical knowledge to the book I feel that co-authorship is not appropriate, especially as the book will be aimed at experts in the area so the content is more important than the communication. However, I would expect more than, say, a mention in an acknowledgements section. 

Does there exist some sort of compromise between co-authorship and an acknowledgement which would better reflect my contribution?
Sometimes the cover gives only the book's title and the name of the principal author but the title page says:

THEORY OF PARTIAL DIFFERENTIAL EQUATIONS

by WILLIAM SHAKESPEARE

with contributions from

Madeleine L'Engle