A tenured professor at my university has hand-copied an out of print textbook in his field. His lectures consist of reading his hand-written notes from the text to his students. He does not deviate from his notes. He does not reveal the source of his notes. He often refers in class to "my notes" and "my lectures." His lectures are the same, year after year. Students do not have to go to class if they get good notes from others who took the course in previous years. Is this plagiarism?
In Germany this is very common in the undergraduate lectures. Usually we were able to go to the library after the first few lectures, look up all the popular books on the subject and find the one that the lecturer was following. You could then sit with it in the lecture and always predict what was said next. Some lecturers told us which books they were following, others had it directly on their desk, some tried to hide it, and some changed mid-term because the next topic was covered better in a different book.
Ultimately this practice helps everyone. A professor is supposed to do research and not spend days preparing an original lecture, and the popular literature on the topic will be much better structured and presented then your professor could probably do. Please be also aware that even when not published as a book, lecturers usually have material collections that they pass on when somebody else is giving their lecture in the next term.
So I am strongly suggesting that you report nothing as you will be probably laughed of by the department while leaving a bad impression.
Edit: There is also a general philosophy of "a lecture is not public", and therefore the usual rules for citations (and also copyright) do not apply.
I have a couple of observations from personal experience. As a grad student, I audited a course in functional analysis. We had a required text for the course. Each day, the professor came into class, opened the book to the appropriate place and started lecturing. In another course, which I actually took, the text was a well-known text on complex projective varieties. After a few days, I realized that what the professor was writing on the board looked awfully familiar, and checked the text (in class) and discovered that he was writing his lecture, verbatim, from the book. In neither case did I have a problem with what was happening. In fact, it made my job easier. As @Terri mentioned in another answer, this allowed me to concentrate on the material, instead of trying to copy it down. I read ahead and knew what was coming. Any useful asides that the prof mentioned were duly noted. Perhaps we were more indulgent of our professors back in the day.
In any event, I have been around the block a few times. There are things that are worth getting upset about. I don't think this is one of them.
Plagiarism refers to passing off others' ideas as one's own. If you are writing a research paper (researcher) or an assignment (student), there is an expectation that you are articulating your own ideas, unless you specify otherwise. If you are conversing with friends over coffee, less so. Class lectures are somewhere in between.
There may be many good reasons to hew extremely closely to the presentation in a book in lectures, especially in certain fields like math. It takes a lot of work to write a compelling set of notes on one's own, and there may be little to gain versus using something already canonical in the field. Of course, parrotting a random book from years ago may also indicate just laziness.
I have definitely at various parts of my academic career lectured "from the [assigned] book", as well as on occasion presented my own material or lectured on part of the material from a different book. I definitely openly mentioned the source, as a natural habit of giving credit where credit is due, and of guiding students where to look (which in the case you mention may be more difficult if the book is out of print, of course). So it does seem weird the professor makes no mention of it, whether it is by design or by accident.
On its own, the use of the words "my notes" or "my lectures" could be using the word "my" as demonstrative, not possessive, i.e. merely which notes he is referring to (the ones he is transmitting to you) rather than taking intellectual credit for them.
All in all, the most likely interpretation seems to be of a pedagogically lazy dinosaur, rather than a nefarious plagiarist, or conversely a brilliant pedagogue bringing you a carefully selected optimal, but unfortunately out of print, presentation of the material.
Finally, putting words in your mouth to be sure, I'm sensing a feeling of injured inequality and hypocrisy here - "how come when we [students] do It it's plagiarism but when a tenured prof does It no-one calls it that". If that's the case, hopefully the distinction regarding assumed level of originality of ideas depending on the context will be helpful to explain it. It's worth also noting that the current level of vigilance about plagiarism on University campuses comes as a direct result of the ease of copying (without attribution) from the internet, with minimal reflection or internalization of the material. Being a bit of an old dinosaur myself, it is just not a theme we thought about as much years ago, when copying something meant manually transcribing it, and so was essentially as much work as rewording it. Still highly relevant for actual research, of course, but less top of mind for lectures, assignments, and the like. So if you pointed out to the professor that you were aware he was presenting "from X", he might well smile warmly at you and say "Oh yes, great that you're aware of X. Such a shame it's no longer in print. In my day, we all learnt [...] by copying out its presentation to understand it" and continue along serenely, with no thought of plagiarism in his mind.