A flipbook is a publishing format in which both the front and back covers book serve as the beginning side of a story; the reader reads one story from the front of the book, and then flips it over to read the second story from the back.
The format is something of a novelty and has so far only been used on a small number of Star Trek publications. In 1996 Boxtree published a flipbook omnibus book of two Malibu DS9 comics; The front side of the book was the story Lightstorm and the reverse side was Terok Nor.
In 2008 Pocket Books release the novel Fearful Symmetry in the flipbook format. The novel featured many plays on the idea of doubles, featuring Kira Nerys and the Obsidian Order operative Iliana Ghemor who was surgically altered to look like her, as well as elements of the mirror universe - Presenting the book in a flipbook format continued this play; side one contained the Kira story, while side two focused on her doppelganger Ghemor.
Additionally the flipbook format has been used for a promotional publication for The Landmark Crossover, a miniseries beginning in 1994 which saw a crossover between Star Trek: The Next Generation and Star Trek: Deep Space Nine by the two different comic publishing companies that licensed the two series, DC Comics and Malibu Comics. The flipbook, distributed at the 1994 San Diego Comic Con, featured one side giving a preview of a Malibu issue of the series, and the reverse side previewing a DC issue.