I wish to start reading novels. I am only interested in cannon material for now (though I do enjoy the cartoon series from the 70's). Alternate timelines are fine though (which by definition could be considered cannon since it is an alternate timeline - as long as there is some explantion for why it is alternate). Or are they all considered non-cannon? What is the official stance from the franchise on the novels, are some endorsed and some merely allowed?
The word "canon" refers only to onscreen material produced by the owners of the Star Trek
franchise (formerly Paramount, now CBS). The novels and comics are licensed tie-ins, all authorized and approved by Paramount/CBS, and they're all obligated to stay consistent with the canon as it stands at the time of publication; but later canon is under no obligation to acknowledge the tie-ins, and thus older books are often contradicted. This is the way most tie-ins work.
However, the books published after the end of their respective series are not as likely to be contradicted, and since there's been no new screen content in the main continuity since 2005, the more recent books are mostly still consistent. But no, they're not canonical. Other tie-ins like the comics and the Star Trek Online
game have their own separate versions of post-series events. There have been Trek tie-ins for over 40 years and they've never followed a single uniform continuity (beyond staying consistent with canon as it stood at the time). That variety of interpretations is part of what makes Trek tie-ins interesting.
After all, none of it is real anyway; the canon is just another bunch of made-up stories. There's no reason stories outside of the canon can't be just as entertaining.