L'Engle's best-known works are divided between the "chronos" and "kairos" series; the former is the framework in which the stories of the Austin family take place, and is presented in a primarily realistic setting, though occasionally with elements that might be regarded as science fiction. The latter is the framework in which the stories of the Murry and O'Keefe families take place, and is presented sometimes in a realistic setting and sometimes in a more fantastic or magical milieu. Generally speaking, the more realistic kairos material is found in the O'Keefe stories, which deal with the second generation characters. However, the Murry-O'Keefe and Austin families should not be regarded as living in separate worlds, because several characters cross over between them, and historical events are also shared.
In addition to novels and poetry, L'Engle wrote many nonfiction works, including the autobiographical Crosswicks Journals and other explorations of the subjects of faith and art. For L'Engle, who wrote repeatedly about "story as truth," the distinction between fiction and memoir was sometimes blurred. Real events from her life and family history made their way into some of her novels, while fictional elements, such as assumed names for people and places, can be found in her published journals.
A theme often implied and occasionally explicit in L'Engle's works is that the phenomena that people call religion, science and magic are simply different aspects of a single seamless reality; a similar theme may arguably be discerned in the fiction works of Diane Duane.