HarperCollins has the C. S. Lewis's Space Trilogy on sale for $1.99 each right now (or $1.39 each at Books on Board). Price should be good at any of your favorite retailers.
Out of the Silent Planet Amazon
That Hideous Strength Amazon
Just as readers have been transfixed by the stories, characters, and deeper meanings of Lewis's timeless tales in The Chronicles of Narnia, most find this same allure in his classic Space Trilogy. In these fantasy stories for adults, we encounter, once again, magical creatures, a world of wonders, epic battles, and revelations of transcendent truths.