I am writing a Young Adult novel with narrative in the past tense however I want to include some words such as now, this, tonight, etc that indicate present but don't know if I can. However, I am reading many YA novels that do. For example, "She lived in Seattle now." "Now, I didn't know what to think." "His appearance was so altered, now dressed in... ". "I was now in a boat.'"Tomorrow would be a nightmare." Should I remove all these references to the present to make the grammar correct. Or should I keep them as some authors are doing?
Those are all correct, and not just in YA. Thankfully grammar has a lot of room for nuance when it comes to tense. As paradoxical as it seems, words like "this" are often preferable to words like "that" in past tense. If I read "But that trip was way different" during a portion of the narrative in which the narrator was on a trip, I would start wondering if the narrator was referring to the trip he/she was currently on or some other previous trip. The "now" or "this" or "here" refers to the moment in the past currently being recounted in the story.
For example, take this hypothetical passage: "Two years after the breakup, I looked Anna up on Facebook. She had lived in Portland, but she lived in Seattle now." Anna's living in Portland and Anna's living in Seattle are both past events, so they're in the past tense. Maybe by the narrator's present Anna lives in San Francisco. But from the point of view of the moment currently being recounted in the narrative -- in this case, two years after the breakup -- only Anna's living in Portland is the past. Anna's living in Seattle is the present of that moment, so it's okay to use words like "now."
The only thing you wouldn't want to say is something like "She had lived in Seattle now." That would be contradictory and confusing because the "had lived" implies that she lived in Seattle before the moment being recounted, while the now implies that she lived in Seattle during the moment being recounted. But "She lived in Seattle now" is fine.
By the way, to back up my statement about this being extremely widespread accepted usage far beyond YA, here's a passage from Chinua Achebe's "Things Fall Apart," high literature by most anyone's definition: "As the years of exile passed one by one it seemed to him that his chi might now be making amends for the past disaster. ... He sent for the five sons and they came and sat in his obi. The youngest of them was four years old." (p.172) The youngest son is four in the moment currently being recounted in the narrative, just as his chi seems to now be making amends for the past disaster in the moment currently being recounted in the narrative.