I completely agree
the problem for me began about 15 or so years ago when a new book Id just gotten , of the format you mention
seemed to contained < 50 % new materiel. I checked with the books I already had.
back then, I was able to call them, they answered the actual phone in Brookline MA and admitted that 40 % or so of in the book I had in hand
had previously been published in their previous books.
and no, they failed to disclose that then, nor do they seem to bother now.