I realise similar questions have been asked previously, but I believe the circumstances here differ in some respects.
My former boss and I worked as practitioners in a particular field for several years, and I was invited to work as a research assistant to do a qualitative assessment of the work we did. We are now working together on a book on the subject. Over the years, I believe we have developed a strong working relationship based on mutual respect, despite his many more years of senior-level experience.
So far, I have written the majority of the work, including background and empirical material. I have also done an extensive literature review and provided a draft theoretical framework which I have begun to connect to the empirical study. Roughly speaking, I have written over 25,000 words of my own, and highlighted potential conclusions including my reasoning. He has written about 2,000.
I recently asked my boss if I would be included as coauthor, but was given the answer that the publisher might not accept a coauthor without a post-graduate degree. I previously wrote an article together with this person on a different topic, and was given the same reasoning that the journal would only accept my boss as the sole author. I was instead included in the acknowledgments. Having researched the publisher, I can see that they may reject the book if the author is not deemed "qualified", for which previous publication is an expectation. I am certain that he will be deemed qualified, but I am unsure how they would look upon me. I am currently completing a Master's degree but won't be finished until next year, at which point the book might be ready. I did, however, work extensively as a practitioner on the topic, in a supporting role to my boss.
I do not necessarily expect co-authorship, but surely there is something such as supporting author or another title that is more appropriate than a mere acknowledgment?
Should I accept the reasoning that the publisher might reject the book proposal? What can I expect in terms of authorship acknowledgment? Should I put my foot down and demand that publication is put on hold until I have completed my Master's degree and published something?
Thank you so much everyone for your insightful answers. They've been helpful in understanding the typical modus operandi of publishers and confirmed my own grasp of the situation.
I have made it clear to my boss that I expect coauthorship, and I will have to wait and see what he replies. Will come back to you with the results.
EDIT: Since raising the issue with my boss, I have been included as cowriter with no damage to our professional relationship. In fact, my contribution to the book has, as a result of the methodology we've chosen, risen to be even more substantially acknowledged. Many thanks for your answers and help, truly insightful. We will see if there is any push-back from the publisher, whose answers I will make sure to verify for myself.