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.

I have had my own work plagiarized and I know it does not feel good, in fact I was quite upset when it happened. In that case the plagiarism was pretty extensive and clearly done with malicious intent (by a graduate student at another institution), and I acted to inform the relevant people. I also had a mild concern at the time that the timeline of events might lead others to conclude erroneously that it was me who plagiarized from the student rather than the other way around.

In your case, however, it sounds like the incident is really too trivial to risk hurting your good relationship with these researchers over. In a comment you say the book contains many errors and there are obvious signs that this was nothing more than sloppiness on the part of the authors. And it sounds like there is no concern you will be mistaken as the plagiarist. So it’s not clear to me what exactly there is to be gained from correcting the mistake in a practical sense, other than to address your own feeling of injustice (which as I said I completely understand and absolutely do not mean to belittle). I assume that these two passages consist of ordinary, everyday text rather than some masterpiece of human ingenuity that will be remembered for many centuries.

My suggestion is to leave it to readers of the book to draw their own conclusions about the professionalism of these authors. If the sloppiness is that obvious, I doubt anyone will be too surprised or shocked to find two unattributed passages in addition to all the other errors. And it is generally not your responsibility to help sloppy scholars improve their work. So, as much as it may be emotionally difficult to do, the best course of action may be simply to do nothing.