I just ran a search on all of Jim Butcher's Dresden Files for the '?!' mark. Why this series in particular?
- It's modern
- Stylistically, I expected to find '?!' there.
- I had it on my computer, so I could Ctrl+f
Here are the results: in 17 books (15 novels + 2 short stories collections) the punctuation mark '?!' appears 11 times.
What does it mean?
Even in modern literature that doesn't take itself too seriously, '?!' is extremely rare. You are right to be asking the question. I suppose other, more verbose methods are used to convey the same effect in other literature. Nonetheless, sometimes '?!' is the most effective tool and the right tool to be used.
“Fuck!” Thomas snarled. I looked up to see him stagger, holding on to the boat’s wheel with his right hand, his face twisted in pain. He’d taken a bullet in his left arm, just above the elbow, and he held it clenched in tight against his body, teeth bared. Slightly too pale blood trickled down his elbow and dribbled to the deck. “Plan B, Harry! Where the hell is plan B?!” (Jim Butcher, Cold Days, chapter 18)
Like you say, no other punctuation mark would give quite the same effect, and the scene is tense and fast-paced, so one wouldn't want to get more verbose instead.