How to address having been plagiarized by senior scholars without ruining the relationship
It is difficult to say if this was intentional or only an oversight - though an improper one. But the fact that they cite you generally, seems to imply sloppiness, not malice. Ask for a correction. 

I would simply point out the passages to them and provide them a proper citation for them. Since this is a book, it will likely go through several printings and it may be possible for the future printings to be corrected. Published books often come with an errata page, usually online these days. 

You don't need to make accusations, just point out the correction. 

If you get a poor response from the authors, then try to contact the publisher, starting with just the same information and request. 

But if you get a favorable response, then thank them and, if you desire, say positive things about their work and point again to your own. From a bad situation it might turn in to a good one. But that assumes good will.