Lewis's shorter works were generally originally composed as speeches or as articles for periodicals. Various sets of them were collected and published in book form both during his life and after his death. Trying to determine what works are in what collections is difficult - most works appear in more than one collection, some works appear under more than one title, and some collections appear under more than one title.
To aid readers, in this review I've listed the works in this collection, with notes indicating other collections they have appeared in. Where a work has appeared under more than one title, I give both titles separated by a slash.
Table of Contents:
Preface, by Walter Hooper (4)
"Christianity and Literature" (1), (2), (4)
"Christianity and Culture" (2), (4)
"Religion: Reality or Substitute" (2), (4)
"On Ethics" (2), (4)
"De Futilitate" (2), (4)
"The Poison of Subjectivism" (2), (4)
"The Funeral of a Great Myth" (2), (4)
"On Church Music" (2)
"Historicism" (2), (3), (4)
"The Psalms" (2), (4)
"The Language of Religion" (2), (4)
"Petitionary Prayer: A Problem Without an Answer" (2), (4)
"Fern-Seed and Elephants" / "Modern Theology and Biblical Criticism" (2), (3), (4)
"The Seeing Eye" / "Onward Christian Spacemen" (2), (4)
(1) also published in "Rehabilitations and Other Essays"
(2) also published in "Essay Collection and Other Short Pieces"
(3) also published in "Fern-Seed and Elephants and Other Essays"
(4) also published in "The Seeing Eye and Other Selected Essays from Christian Reflections"
If you are interested in Lewis's shorter works, my best advice is to get "Essay Collection and Other Short Pieces", which, as of the time of this writing, is available from Amazon UK but not Amazon US. That collection consists of about 130 short works by Lewis. The works in that collection are mostly, but not exclusively, Christian.
If your interest in Lewis's shorter works is restricted to those on Christianity, and your budget or enthusiasm does not run to "Essay Collection and Other Short Pieces", then my second-best advice is to get any or all of the following (they don't overlap significantly, and between them they include most of Lewis's shorter Christian writings):
"God in the Dock - Essays on Theology and Ethics"*
"The World's Last Night and Other Essays"
"The Weight of Glory and Other Addresses"
* Be careful - there is a UK Fontana paperback lurking about called "God in the Dock - Essays on Theology" that is substantially shorter than the "God in the Dock - Essays on Theology and Ethics" collection. A full version of "God in the Dock - Essays on Theology and Ethics" was published in the UK under the title "Undeceptions - Essays on Theology and Ethics".