I came across plagiarism. Whom should I inform if at all? 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.

Should I let someone know about it? If yes, whom?
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.