Martin Gardner's Annotated Alice
Martin's best seller by far was The Annotated Alice (Clarkson N. Potter, 1960, 351 pages) and its later incarnations. It included the original text of Lewis Carroll's "literary nonsense" books Alice's Adventures in Wonderland (1865), and Through the Looking Glass (1871), along with extensive commentary on both, and background information on the characters.
In a 2005 interview with Don Albers, Martin commented, "[I]t has sold more than a million copies if you include paperbacks and translations. It has never been out of print." It also launched a genre, with W. S. Baring-Gould, Isaac Asimov, and other authors soon pursuing similar projects. Its success also led to Martin publishing a whole series of annotated books.
Thirty years later, Martin published a sequel, More Annotated Alice (Random House, 1990). The two books were in due course updated and combined as The Complete Annotated Alice (Voyager, 1991), and more definitively as Annotated Alice: The Definitive Edition (W. W. Norton, 1999).
The Universe in a Handkerchief: Lewis Carroll's Mathematical Recreations, Games, Puzzles, and Word Plays (Copernicus/Springer-Verlag, 1996, 10 + 158 pages) surveys the topics indicated by its title in the chapters Fiction and Verse, The Diaries, Letters, Books and Articles, Miscellaneous Amusements, Doublets, and Pamphlets on Games.
Here's a 1979 interview by Jan Susina with Martin about his Alice interests and writings (from The Five Owls, Jan/Feb 2000).
The next book takes as its starting point Martin's Alice in Wonderland interests and scholarship, but contains much material unrelated to Alice:
A Bouquet for the Gardener (Lewis Carroll Society of North America, 2011, edited by Mark Burstein) contained a contribution from Martin himself.