I read this in the mid-'90s; the book was from my father's collection and was showing its age. The main characters had to leave Earth for some reason, and wouldn't be pursued because of Earth attitudes toward space travel (or maybe atomic energy). They ended up visiting a lot of the planets, or their moons (in the case of the gas giants) and on each planet met some sort of weird alien.
Early on, one alien that was what we'd now call a xenomorph taught them telepathy (which teaching caused a headache); one of the worlds had a nuisance vermin (rabbit equivalent) that could become invisible and make other things invisible by having enough of them sitting between the viewer and the object; one world the aliens were centaur-like; one world built their civilization with almost no metal, and the main characters escaped imprisonment by convincing their jailers to provide them with materials that (unknown to them) would embrittle the walls so that they would crumble at a kick; and there were other fantastic portrayals of that sort.
I don't remember the author's name, except that I remembered seeing it as a not-an-author.
I'm fairly sure it was one side of an Ace Double, but I have no recollection of the 'flip'.