I have been reading a sample PDF of a book by a professor at a research university published in 2007 on the publisher’s website. For some reason the book’s title was not included in the PDF’s footer (as it should be) so I had to google a passage from the book with the hope that Google Books would provide me with the title. I was quite surprised when it provided me with two titles.
There was a second book by a completely different author (an engineer with a PhD who works in the industry) published in 2015. Not only was the passage there but the whole paragraph (possibly more) verbatim. And even though the second book had tons of references, the first book wasn’t among them.
I would suggest contacting the original author (A1) to let them know about the (probable) plagiarism, rather than directly contacting the second author/publisher (A2/P2). This way:
At the point where you get in touch with a stranger, you are not accusing them of anything, so communication is much less loaded/constrained.
If A1 tells you they have authorised the re-use, then you know it’s OK; whereas if you contact A2/P2 and they say it was authorised, it’s hard to be sure you can trust them (since their ethics are already in question).
If the re-use is indeed not authorised, then A1 is the person with the best legal and moral authority to contact the publishers P2 and make this allegation.
Not to copy and paste my answer from another question, but...
You should contact the editor/publisher of the book which plagiarized (book 2), present your claims, and be prepared to submit evidence. If there's no response, contact the editor/publisher of the original book (book 1) - they may be able to use legal means to force the matter.