(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 form for use on portable electronic devices. Most notably, the individual stories of the Star Trek: Starfleet Corps of Engineers series are first released in eBook format, then collected and reprinted in standard paperback format.
The Magic of Tribbles: The Making of Trials and Tribble-ations is one of very few reference books to have been only released in eBook form. The first chapter of the Star Trek: Deep Space Nine Companion was also released, as promotion for the full print release. An eBook release followed in 2011 of the third edition of the Star Trek Encyclopedia.