In her latest two S-F concoctions [Andre Norton] shows the varied possibilities of inventiveness within her genre. [Quest Crosstime] is a political, villains-and-good-guys adventure, earthbound, with ingenious development of time-travel…. Particularly imaginative are the pictures of worlds, like E625, in which crucial alterations of events change the future altogether, so that the United States never comes into being, pre-empted by a part-Mayan, part-English civilization. This intriguing adventure will lure girls into S-F fandom since the lead characters are mind-linked twin girls…. (p. 636)

Whereas adventure is paramount in Quest Crosstime, atmosphere dominates The X Factor…. The author goes far beyond good plotting and peopling of fiction to construct a fantastic never-never-land of new color, new sound, new motion. Readers feel truly like off-worlders with off-world eyes suddenly gazing for the first time on obscene creatures of slime, or enchanting red and silver woodlands, or the shadow people of Xcothal. (pp. 636-37)

Jane Manthorne, in The Horn Book Magazine (copyright © 1965, by The Horn Book, Inc., Boston), December, 1965.