Perelandra was published in 1943, one year after A Preface to Paradise Lost, and deals with many of the same issues: the value of hierarchy, the dullness of Satan, and the nature of unfallen sexuality, for instance. To an extent, it can be viewed as a commentary on Milton's poem, but a commentary which is intelligible to a reader ignorant of the original.
Lewis's description of Perelandra's environment and rotation period is, of course, inconsistent with the actual conditions on Venus, but astronomical observation at the time of writing of the novel had not yet positively determined this to be the case. A Venus largely or wholly covered by a worldwide ocean was a common theme in science fiction works of the time—a logical, though eventually proven erroneous, inference from the planet's thick cloud cover that it had constant rainfall and ubiquitous oceans.
The names of the Adam and Eve characters of Venus, Tor and Tinidril, were consciously adapted by Lewis from those of Tuor and Idril, created by his friend JRR Tolkien.
The third volume of the trilogy, That Hideous Strength, is set on Earth and, perhaps inevitably, has rather a different tone from the prior two volumes; Ransom is a key character but is "off-stage" for much of the action.