I go to Barns and Noble at least once a week, and stop by the Sci-Fiction section looking for new Trek books, I have never considered buying the Novels. They were written after the series were completed. What is your opinion of the Novels, can they be considered extensions of the actual series or are they just books that are assuming what may or may not have taken place in the televised series???
Both. There are many novels that take place during the series and can be seen as having taken place in between episodes of each series. But there are also many that take place after the series end.
The most notable of these is the DS9 "Relaunch" which basically consists of "Season 8" of the series and is now in the beginning of "Season 9". It follows the adventures of Captain Kira and the rest of the DS9 crew both old and new.
Similarly there is the TNG "Relaunch" which takes up where Star Trek: Nemesis and the novel mini-series "A Time To..." left off. It continuies the adventures of the Enterprise-E after the departure of Riker, Troi, and Data. There is also a novel series called "Titan" which stars Riker and Troi on their new ship.
There is also the Voyager "Relaunch" which takes up where the series finale left off with new Voyager adventures back in the Alpha Quadrant.
Finally there is the Enterprise "Relaunch" which picks up where "Demons/Terra Prime" left off, showing a non-holodeck version of the events of "These Are the Voyages..." and continuing on from there.
All of these novel lines and a few more form a vast continuity which could be described to a newcomer as a "Star Trek Expanded Universe", though that term is never actually used.
Since there is zero chance of any of these series returning to the screen, none of this can be contradicted by future productions. So the idea that this is somehow "less official", or "not-real Star Trek" is absurd.
I'd say swing into the Trek Liturature forum for more information and then dive in to the vast sea that is Trek Lit.