The most effective length of books varies, of course, and you should write the book to the length you think the story needs. But if later you decide for whatever reason that the book would be more effective longer or shorter, here are some tips to make that easier.
Longer: Today's new adult readers read 700-page books (okay, all starring Harry Potter) when they were 10 years old. So they won't be intimidated by length. Whether a long book would be more marketable as one volume or two or three is a discussion for another time. And you never want to stretch a short plot into a long book—the threads get pretty frayed then!
However, you might be looking at an older category book (written for a line like Harlequin Intrigue or Berkley Prime Crime), and considering expanding it to single title length, which is generally between 75K and 100K words. You don't want to just add more words—that's like drinking milkshakes to bulk up your muscles. Add length by adding complexity.
One way to do that is to add an additional major plot. Shorter books naturally tend to have a single central plot (a romance will have a central romantic plot, a mystery a central crime plot), and any other storyline is generally reduced to subplot level, starting further into the book and being resolved earlier than the main plot. Rather than adding more subplots—a lot of subplots often leads to confusion—beef up the subplot that is most connected to the main character's emotional or psychological journey and make it clearly support the main plot.
Character journey: In this romance, the heroine's father died when she was a child, and her mother married again and moved her far away from dad's family. The subplot might be about reconnecting with the family—phone calls, setting up a visit—but it's resolved quickly because the original purpose was just to get her back to dad's hometown where she meets the hero.
What's her emotional journey? She's moving perhaps from a fear of abandonment to trust? That's a good journey for a romance, as learning to trust is a major step on the way to love. To lengthen the book, consider having that subplot of reconciling with the family take place over most of the time of the book. That will mean adding conflict—that is, to make it a full plot, you can't resolve it in Chapter 3 when she starts interacting with the hero.
If you want to have her start with fear of abandonment, you could have her—instead of reconnecting –before- she comes to town—keep quiet about her identity, come to town, and scope out the family before revealing herself. After all, if she's afraid of abandonment, she might think the family kind of abandoned her by losing contact. So she could come to town, planning to observe her relatives in secret before deciding whether to approach them. This would add a motif of disguise that could complicate the budding romance (is she open with him about her real identity and connection to the town?), and if you make him have some issue with the family (business or political rivalry, maybe), this would add a further conflict to the romantic plot.
That is, add conflict, not words. This will affect the entire story, of course, and require changes in most existing scenes and additions of new scenes, so this isn't a task to take on lightly. But it's a choice we witness a lot these days as authors go back to perfectly good category books that didn't sell back when category was king. Now they have a real option—keep it short and try to sell it as is, or add 10-30K words and sell it as a single-title.
Just as common these days is the decision to shorten a book. This is more the norm for me as I always write too much and have to cut on the order of 35K words just to get it down from "epic" length to "single-title" length. So I know there are different kinds of "too long." Take a few days and read over the book as it is. Is the plot too long and complex for the length you want? Or (as always in my case) are there the right number of scenes, but the scenes themselves are too long?
Diagnose the problem before you start cutting! You don't want to end up just cutting words when you really would do better to cut out a subplot or combine several scenes. It can really help just to boil the plot down to an outline with a line or two of summary for each scene.
See if there are some scenes where only one plot-important thing happens, or none at all. For example, I've edited books where the only really essential event is that the sleuth finds a clue. In that case, could that paragraph or page about finding the clue be moved into the previous or subsequent scene, so that one scene can be eliminated? What I like to do then is find whatever in that scene is important (either to the plot or to the author—you know what I mean, the perfect sentence of description, a great interchange of dialogue) and start stripping away everything else in the scene. What's essential and/or worth keeping? Move that into an adjacent scene.
Also look for scenes that basically do the same thing (like the hero twice encounters his prime suspect downtown) without any escalation of conflict. You might not need both those scenes. Another place you might find extraneous scenes is in the beginning. We often write long openings because we're trying to get to know the story and the world, but that might mean that we start a couple scenes before the story really begins. Leisurely openings can be interesting, but if you're trying to trim your book, you probably can't afford extraneous scenes.
Now if you're like me, you might have just the right number of scenes, but spend too much time on each. When I decide to cut the length of scenes, I start at the beginning. Often I can cut a couple paragraphs right from the first page of the scene. I also replace long explanations of motivation or action with a "narrative bridge" of a few words, like "She gave up, too exhausted to continue." I also look for redundancy, where I show something in the action, and then explain it again in introspection—I cut out the introspection unless there's no way for the reader to get the point of the action.
Trimming like this can really improve the pacing as there aren't pages of narration between important events. (By the way, it's always painful for me to delete my passages, so I just cut them and paste them into a "cut file," just so I'll have them if I need them. That makes it easier!)
To cut radically, as when you are trying to turn a novel into a novella, you probably have to get into the very structure of the plot and simplify, first by cutting out a subplot or two, and second by streamlining the conflict. The main conflict might have to be simplified so that it can plausibly be set up, intensified, and resolved in 150 pages. Think about diminishing the internal conflict. In a longer book, perhaps a man can get over being unjustly imprisoned for a crime he didn't commit, but for a shorter book, you could diminish that to an unjust accusation without any imprisonment (maybe he got off because of a hung jury), so that he just has to vindicate himself, not deal with the ramifications of having been in prison.
Reinvention takes re-imagining. But I've found it much easier to do when I am clear about what the book IS and what I want it to be. Just asking the questions about whether I need to change the plot or just the scenes gets me half the way to determining what reinvention will transform this story and make it new.
What reinvention situations have you encountered? What did you do?