I have completed a novel manuscript that is on the shorter side at 66 000 words. Initially I contemplated approaching agents and publishers who work with Young Adult titles, as it does have a teenage protagonist, but after sending it to a few writerly friends and colleagues (I'm an English teacher) for feedback, I was told what I think I already knew deep down: it's not really a YA novel. This leaves me in a bit of a dilemma when it comes to the length. From all that I have read online, agents are reluctant to take on books this short, especially if they aren't literary fiction and are coming from a first-time author. But I've also read that nobody wants to read filler scenes and you shouldn't bulk out a work more than is necessary to tell the story. Do you think it would be wise to try to add 10-15 000 words, to start sending queries about the manuscript at its current length, or is there something else I should consider? Or should I put this one away to gather dust with my first few (terrible) novels and focus on the next one?
Sixty-six thousand words isn't short for grown-ups' fiction; it's just fine. (At least, for Australia - I don't know which country you're in.) I don't know which websites you've been reading that say it isn't, but it wasn't this website ...
'Ideal' word counts exist largely because of the costing of the book: the publisher has to buy a certain amount of paper to produce a book that will turn out to be X pages in length; they want to charge $Y for that book and the cost of paper therefore affects their margin on the book, which is likely to be very slim, because most of them are. Your word count is within the range of what's acceptable for 'general fiction' - a publisher could make their costing work on that. (It's slightly larger than most YA titles, though, in case you're curious.) The only genre where it might be problematic is fantasy, where readers expect the books to be about double that length.
Ultimately, though, you have to write the story and just let the word count sort itself out. If the story feels like it's too fat, trim it; if it could use more scenes, add them. But just let the story determine what it needs and work from there.
*This last bit was added for dramatic effect.