Hi,
I am not saying that my approach is right and I don’t have much experience but the thought that occurred to me is that most importantly I think she would have to see the beauty in Shabbos in order for her to want to observe it more. I don’t really have any suggestions on specific books but sometimes a good book that explains the beauty and importance of Shabbos and what it means in terms of it being a special day where one can connect to his/her creator on a higher level. Also I would suggest if applicable, if she can somehow get involved with a Shabbos program that deals with children, i.e an oneg Shabbos or b’nos etc… where she will see the childrens joy in partaking in the beauty of Shabbos. Once the connection is in place I think things will fall more into place and she might be more interested in observing shabbos. Also I would suggest that as much as possible if she could somehow be invited to other peoples houses for Shabbos meals it would probably be good for her. Once again I don’t really have any experience just my thought. Hatzlacha.