Mostly Britain, though Maxwell did also very briefly control both Prentice Hall and the American side of Macmillan from 1988/9.

Some of the (rather convoluted) history is at https://en.wikipedia.org/wiki/Macmillan_Inc.#Mergers_and_end - but there's not much evidence for any conspiracy GP may have been hinting at.

(Cap'n Bob Maxwell was essentially the left-wing Rupert Murdoch - big on tabloid journalism and building media empires through fair means and foul. A hugely controversial character, but his involvement in textbook publishing was fairly incidental)