If this was one of my snarky movie reviews, the tag line would be, "It ain't over till the dead lady sings", but I like books in general, and I like Mary Gentle's books in particular, so I'll hold that one back against the possibility that one day reality breaks and someone figures out how to make a movie out of a Mary Gentle book.
Weirdly, if they did, Black Opera might be the easiest place to start. It's got a pretty straightforward plot and we see everything through one pair of eyes. And there's nothing outright unnatural in it; if you had the wherewithal to make a costume drama in Napoleonic times and the spare money for a halfway convincing volcanic eruption, you're sorted. I don't know where you'd even start if you wanted to make a film out of the gloriously bonkers Grunts, or the intricately strange Ash, to pick my two favourites. But in a world which has seen Abraham Lincoln Vampire Hunter, there's no reason why you couldn't make a movie about an attempt to stave off the invocation of the devil on earth by using an opera to trigger the eruption of Mount Vesuvius. In post-Napoleonic Naples.
Since this is Mary Gentle, it rapidly becomes clear that this isn't quite our post-Napoleonic Naples. For a start, it's not really post-Napoleonic. Napoleon is still in charge of a huge swathe of Europe, since Waterloo worked out a bit better for him than it did in history (and Borodino worked out a lot worse, as we learn in a fun bit of backstory for one of the other characters). When I started picking up on those little clues, I began to wonder if this was going to be like Ash, where an apparently straightforward book about condottieri got gradually more and more divorced from actual history.
About half-way through the book it began to feel more like a Tim Powers book than anything else; it has a bonkers quest to avert armageddon through the power of art, which is a very Powers preoccupation, and it was putting its characters through the wringer something wicked, which is another Powers habit. And it was cluttered with dead people playing an active role in the world of the living. None of these are bad things, and none are done badly, but they're Powers things, more than they're Gentle things, and I was constantly feeling a little adrift from my expectations.
The plot, as I said, is pretty simple as these things go; nut cases set out to trigger volcano using the power of opera so as to summon the devil and end the world; good guys set out to stop this by setting up their own opera to drown them out. Gentle being who she is, the pacing and resolution are clever, logical and unconventional, with things coming to a head when you don't expect them to. But that's par for the course; what's impressive is that she wrote a more than 600 page novel, most of which is about the intrigues and headaches of trying to write an opera. There aren't that many big thrills or adventures all the way through the middle; there's just a bunch of people doing something difficult and creative, and somehow Gentle manages to make that gripping. That's a good writer; someone who can make me interested in something that bores the pants off me in real life.
All that makes Black Opera a good book, but if I was sending people off to start on her work, it's not where I'd begin. Her other work is just more fun; there's more new ideas and clever things in some of her earlier books. Grunts is huge fun in the most appalling taste, a fantasy novel where the bad guys are not bumbling idiots just waiting for the heroes to finish their journey and wipe them out. In Grunts, the bad guys have a clue, and an already good idea kicks it up a notch when a band of genre-savvy Orcs trip over a cache of Vietnam weaponry. Not only can they figure out how to use it, but they start to act like Vietnam War infantry grunts into the bargain. Grunts reads like an immensely long pub wrap-up of the world's most depraved D&D game, which I gather is not too far off how it came to be written in the first place. Slightly more seriously, Ash was Gentle's first step into her continuing habit of writing books which are not quite history and not quite fantasy, but somewhere in between; our own world's history kicked slightly adrift with a couple of counterfactuals and a little bit of magic and then left to run on logically from there. Ash starts out as a hard-headed take on condottiere romances like The White Company, and swerves off rapidly until we're in a world where Carthage is not only still a thing, but it's invading the Holy Roman Empire and kicking its arse hard. Which is only the beginning of the crazy. That's where I'd start; only when you've seen something that left-field can you start to appreciate how the more controlled and restrained Black Opera is a demonstration of how well Gentle can write, as opposed to how many good ideas she can have.