I am aware of a professor, who wrote two published books that had to be corrected. On the publisher's website it states that these two books had been corrected due to "inadequate referencing". Is inadequate referencing essentially a euphemism for plagiarism? Or is it possible that inadequate referencing can really be a lesser sort of offence?
I think that's a rather unkind interpretation of what happened here. Books aren't the same as research articles - especially for text books, it is par for the course that large swaths (most?) of the book are not actually about the author's own ideas.
Further, extremely detailed referencing can easily reduce the readability of the book, so oftentimes book authors are given a bit more leeway than what would be acceptable in a research article.
What I assumed happened here is that either some references are simply missing (as Solar Mike said), or that it has been later on determined that some parts of the book should really reference more explicitly where the respective content came from (either because the original author complained or because the book author decided that some more references would help a reader find additional information). I would not assume this to mean that the book author blatantly copied material from somewhere (this, presumably, would not lead to a correction but to withdrawing the entire book, because it also sounds like a copyright nightmare for the publisher).
I'm guessing it's a euphemism for dubious content, rather than plagiarism.
While books should cite their sources, it is not necessary to cite every single fact. The book's author (who is an expert on the topic) will justify or prove most facts in the course of the discussion. It's good practice to include a bibliography showing relevant primary sources (e.g., for further reading); however, merely summarizing a published paper in a textbook does not necessarily require a citation (e.g., we do not cite Newton's papers in introductory physics books). Citations are really only needed when facts are asserted without being proven or justified through the narrative.
In this case, the word "inadequate" does make it seem that there was a problem. This could be a euphemism for plagiarism: word-for-word copying, or not meeting the publisher's standards in terms of referencing relevant work. Or, it could be that the author stated a lot of facts without justification or citation, and some of them turned out to be dubious or even wrong. That's my guess.