C.S Lewis is most widely known for his Narnia novels, especially now that three of the series - The Lion, The Witch and the Wardrobe, Prince Caspian and The Voyage of the Dawn Treader - have currently been made into films, with at least two more being planned.
The Cosmic Trilogy
He also wrote another, little known series of books known as The Cosmic Trilogy, consisting of Out of the Silent Planet, Perelandra and That Hideous Strength.
The Cosmic Trilogy was little know even prior to the various cinema and television adaptations of the Narnia books, and I believe rightly so, for the series has what I consider to be a fundamental flaw.
There have been quite a few discussions regarding the Christian symbology displayed in the Narnia series - which symbology I did not even notice as a child, despite having a lot of Christian education, and still did not notice until it was pointed out. It is my own personal opinion, which others may disagree with, that C.S. Lewis's beliefs do not obstruct the story in the books of the Narnia series.
In The Cosmic Trilogy, I believe this is definitely not the case.
This series is a science fiction trilogy. Its writing predates the exploration of any other bodies in the solar system, including the moon, and in fact predates both manned and unmanned space flight, being written in the 30s and 40s. There are therefore many errors made in the descriptions of other planets, such as Mars and Venus, and of space travel itself, although at the time of writing this was all purely theoretical.
This isn't, in itself, the fatal flaw. Many books written in science fiction's Golden Age predated space travel. Books by, to name just a couple, Edgar Rice Burroughs and Robert A Heinlein, depicted planets such as the previously mentioned Mars and Venus in ways that have since been disproven. These books can still be enjoyed for the stories.
The author does not believe mankind should travel beyond Earth's atmosphere, and makes this opinion clear in the series. Interplanetary space is called "God's Quarantine Boundaries." This is what I consider the fatal flaw with the trilogy.
The reason I think this is a fatal flaw is this: the series is fundamentally science fiction, and the main readers of the series would be people interested in science fiction. This is a trilogy about space travel and visiting other planets, a subject which primarily appeals to readers who find this idea appealing, yet has been written by someone who disapproves of the whole idea and believes it to be quite possibly contrary to God's Will.
A series of books written by someone with such fundamentally different beliefs to those of its likely readership has a serious problem, and is unlikely to be enjoyed by the many of those readers.
Read the Narnia books, but if you're interested in science fiction, don't bother reading the Cosmic Trilogy.