I don't think it takes much reading to learn where the middle begins and ends, or that the writer should fill the middle up with, well, stuff. The exact nature of that stuff usually turns out to be “conflict” and plot points and so forth. Talking to writers, reading blogs, and digging through books, the middle always seems to be a mire.
I'll note here that I'm a structuralist. That doesn't mean outliner or seat-of-the-pants writer—I do both, based on the needs of the story at hand. (The Superhero Murders is heavily outlined because after a few attempts to improvise, I found I just didn't have a handle on the drama.) Being a structuralist means drawing on structure to build a story, regardless of whether you do it while your write, before you write, or after you write.
The first question is how to find stuff (i.e., conflicts and plot points) to put into the middle.
The plot points come out of the arc. Who is changing? Who has the opportunity to change, but doesn't? Which incidents will facilitate and highlight that change.
Going beyond that then, one trick I use to get ideas is to list all of the character's resources, and then all of the opposition's resources. Then list limits: where is the line? I've mentioned this before. The resources must be things the character is willing and able to draw upon in the struggle (whether it's a literal fight or, say, conflicting social goals.)
Consider the first Hunger Games movie. (It's been some time since I read the book.) Katniss is not only up against the other kids, but the game keepers as well. What can the keepers bring to the fight? A lot. They can use the environment against the kids. They can change the rules at a whim. They can herd the kids to a single point. They can up the pressure by releasing monsters into the arena. And they do all these things, but they also have limits: the games must entertain, and they must have a winner. That last point is used against them in the end.
Once you list what each character can bring to the table, review and look for any characters with weak a weak hand—the powerless. Those guys will either need to be trimmed or given an ace up their sleeve.
Why do this? Because when you reach Act III, you want the reader to feel as if each side has offered a good, strong fight. They've played all their cards (except maybe the ace), and it's time for the final showdown where it is about who will give up more and risk more, not who has more cards in their hand.
You can do this at the scene level too.
So that gives a stack of things to work with. Do you use them all? Not quite.
(A friend recently enlightened me on this.)
The next step is to evaluate each conflict that arises from those use of resources. The ones you want to use are the ones that lend to character growth, or character change. Which conflicts will teach skills or drive someone along their arc? The goal isn't conflict for the sake of conflict. The goal is conflict for the sake of the character's or the story's development.
So list resources to get ideas. Evaluate them in the light of character arcs. If you still find the middle a slog, I would venture that you're missing something in terms of arc or character passion.
Writer of speculative fiction. Current work in progress is an untitled dark fantasy novella.