So "Pocket" essentially licensed rights to create the continuous series of DS9 books?
Are there other series that contradict this one are is only this one allowed?
The publisher Simon & Schuster, whose imprints include Pocket Books and Gallery Books, has an exclusive license from CBS Studios (which is owned by the same conglomerate) to publish prose fiction based on Star Trek
. It's had that license since 1981, when CBS Studios was still called Paramount (to put it simply). That means that, most of the time, it's been the only company allowed to publish novels or stories based on ST, although there have been a couple of small exceptions; briefly in the late '90s, the revived Amazing Stories
magazine obtained a license to publish short fiction based on The Next Generation
, and just recently Star Trek Magazine
published an original short story based on the Star Trek Online
computer game. Pocket/S&S used to have the license to ST nonfiction as well, but now that license has gone elsewhere.
There is also a separate license to publish ST fiction in comic-book form, and this has been held by many publishers over the decades: Gold Key in the '60s and '70s, Marvel Comics following the release of the first movie in 1979 (plus a separate daily comic strip from the LA Times Syndicate), DC Comics in the '80s and '90s, then Malibu (exclusively DS9), then Marvel, then Wildstorm. IDW has held the license to do TOS, TNG, and eventually DS9-based comics for the past several years, and for a while TokyoPop had its own license to do TOS and TNG comics in manga format, but they're out of business now so it's just IDW anymore.
As far as contradiction goes, all tie-ins have to stay consistent with the canon (the shows and movies), but there's no requirement that all Trek tie-ins be consistent with each other, even within a single publisher's line. Pocket maintains an overall continuity among most of its novels as a matter of authorial and editorial choice, but there have also been novels that were intentionally kept separate from that continuity. And IDW has no overall continuity among its comics; all the ones written and illustrated by John Byrne are interconnected with one another, but the rest are all standalones and aren't always consistent with one another, and a couple have even tied into the Pocket continuity while others have conflicted with it.