The first ebook only releases were the
Corps of Engineers/SCE books. There were 74 in the series, running from 2000 to 2007. All except the last eight have now be collected in paperback omnibuses.
Towards the end of the
SCE run there were also two miniseries, which took six month chunks out of the usual
SCE monthly release schedule.
TOS's
Mere Anarchy spans the entire
TOS timeline from TV to movie eras, and is also available in a paperback omnibus.
TNG's
Slings and Arrows takes place in the year before
First Contact, the six novellas in this series have not yet been omnibus...ified...
Then with the end of
Slings and Arrows there were no more ebooks. I think
Trek was a bit ahead of its time. Although the decision to stop came oddly as the new wave of ebook readers were really starting to get popular.
After a few years ebook only stories returned! One a year so far since 2011, each tied to different series:
Typhon Pact: The Struggle Within, was an extra story released shortly after the initial
Typhon Pact miniseries.
Vanguard: In Tempest's Wake, a "coda" to the
Vanguard series.
And coming this year,
TNG: The Stuff of Dreams, set after the
Cold Equations trilogy.
Also out this month, are
three new ebook omnibuses with no paperback equivalent (well, two of them don't at least), which collect together several recently released novels.