For anybody not willing to burden their text with full-on quotations, but to avoid such plagiarism claims:

It can be as unobtrusive as putting a simple footnote after each of those paraphrased blocks of text, thus only appearing as tiny numbers in the text.

If Klein had just done that, he would have escaped this kind of accusation, while not changing a single character of his books as they are currently written -- except for the tiny footnote numbers. The footnotes contents can also be gathered at the end of the book, and this is completely normal.

And there is nothing wrong with a text full of footnotes. That's really the standard, actually, in books targeting advanced readers, such as students.