All Judgement Fled (1969) - a "standalone" novel by James White. This was serialized in IF from December 1967 to February 1968. [I read this decades ago and liked it a lot, but now rely on a review from ANALOG December 1970, for a decent summary]:

"A gigantic ship appears out of space, moves into the solar system - and just lies there. Two teams are sent out to make contact: four astronauts and 2 scientists, all under military command. They are an ill-chosen crew, and personal tensions begin to develop, not much helped by the Voice of Authority back on Earth, constantly dinning in their ears. Matters are bad enough before they reach the Ship, but they rapidly get worse. They break in.. begin to explore... then find themselves in open warfare with malignant creatures in the Ship. Are they the voyagers from the stars, or are they food animals, experimental animals, that have escaped and taken over? What is the secret of the Ship's interstellar drive, and how can it be fathomed? Meanwhile, back home, Earth is in the midst of a turmoil brought on by the uncensored broadcasts from the Ship and not unlike the divisions we see around us today. Politics... military protocol... naive liberalism and equally doctrinaire authoritarianism... everyone has to get into the act. Meanwhile five men inside the ship and one marooned in their own vessel are probing and fighting... " [P. Schuyler Miller]