reference or co-author?

[COD]

Hi, I have got a place for a phD at my chosen university but didn't succeed in obtaining a fully funded scholarship because they felt my proposal was a bit too specific. My supervisor says that she is keen to write a book on a topic close to my proposal and if I widen my focus that she will use my research for her book. She mentioned my co-authoring it with her, or crediting me for the contributions to her book. She is a highly respected expert in her field and I am very flattered that she has offered me this opportunity. In fact, she has even said that she will continue to supervise me even though my term extends beyond her retirement! However, to me, there sounds like a big difference between a co-author and a reference, anyone have any views on this? Also, what are the career benefits to me of being involved in such a project?

Thanks!