(written from a Production point of view)
The term eBook is a short form of "electronic book", and refers to any type of electronic publication released to simulate the form of a print publication.
Many Star Trek novels have been released in this format. eBook releases of Star Trek novels concurrent with their print release are now standard practice, and Pocket Books has released its Star Trek back catalog in a number of eBook formats.
Original eBook publications were first trialled by Pocket Books in 2000 with the launch of the Star Trek: Starfleet Corps of Engineers series. These eBook-exclusive novellas were initially intended to have no print release, but were later collected and reprinted in standard paperback format.
At the time, there were no dedicated devices for reading eBooks - the launch of the series was intended to coincide with that of the Microsoft Reader software.   The series would continue for seven years.
Other early examples of Star Trek eBook releases include the hardcover release of the Star Trek: New Frontier novel Stone and Anvil, which included a CD-ROM containing all of the previous books in the series.
In March 2008, it was announced in that original eBook publications from Pocket Books would go on hiatus.  Series editor Keith R.A. DeCandido later noted that the concept "was about ten years ahead of its time" and so failed to find the audience it needed, with SCE being cancelled just as devices such as the Amazon Kindle entered the market. 
Original eBook releases would return in 2011, in the form of one-shot novellas as part of the various ongoing novel series.
Original eBook publications
- Star Trek: Starfleet Corps of Engineers (series)
- Star Trek: Mere Anarchy (miniseries)
- Star Trek: The Next Generation - Slings and Arrows (miniseries)
- Star Trek: Typhon Pact: The Struggle Within
- Star Trek: Vanguard: In Tempest's Wake
- Pocket TNG: The Stuff of Dreams
- Star Trek: Titan: Absent Enemies
- Pocket TOS: Seasons of Light and Darkness
- Pocket TOS: The More Things Change
- Pocket DS9: Lust's Latinum Lost (and Found)
- Pocket TNG: Q Are Cordially Uninvited
Because of their typically image-heavy content, reference books are less often released in eBook format. The Magic of Tribbles: The Making of Trials and Tribble-ations is one of very few to have been released as an eBook exclusive. The first chapter of the Star Trek: Deep Space Nine Companion was also released, as promotion for the full print release. An eBook release of the third edition of the Star Trek Encyclopedia followed in 2011.