Mild spoilers follow
In many ways Hollow was written to target me. The aesthetic, the tone, the setting, the language - I loved them all. But I can't help but feel like the plot (specifically the ending) represent a huge missed opportunity to become a modern cult classic.
The first thing to speak of among that list is the aesthetic. The author has done a remarkable job in striking a dismal, grim-fantasy tone. Intersections between character histories, archaic language, religious intrigue, horror references, monstrous creatures - these come together to weave a potent horror tone. I was a huge fan. Significant care has been taken to maintain a consistent terrible flavour. Plot points weave into the tone just as much as the dialogue, which is variably given in explicitly and sometimes almost crude archaic fashion ('whomst', 'thoust' etc.) It works, although I can see how it might rub some the wrong way.
It is worth mentioning Hieronymus Bosch, whose output very evidently influenced the author and indeed who is explicitly mentioned in the work as a man gifted with maddening visions. It's probably the best way to describe the general tone of the novel without specifics: it's like looking at a particularly haunted Bosch painting and imagining a grim quest attended to by a host of ill-fated bastards within it.
About said bastards next. The novel is roughly divided into three storylines, each of which have mostly consistent but sometimes-changing points of view and converge at the end.
The first concerns a gang of unscrupulous and repulsive mercenaries who have taken up a job that nobody else would dare to take: the delivery of an Oracle to a sacred monastery as a place known as Das Kagel. This is by far the most engaging of the storylines. Each of the characters are fairly thinly developed, but have enough of a personality to feel meaningful; exceptions are Follett, Pearlbinder, and the Kid. The Oracle is a masterpiece of horror-writing. It is something not at all human, carried around in a crate of ice and fed human bones to guide the men. The party has to stop every now and then to perform a ritual they call 'scrying' - they take fresh man-bones and whisper their greatest sins into them to infuse them with terrible energies before feeding the marrow to the Oracle. It goes without saying that this serves as a horrific engine for character exposition and a weighty dose of existential horror. Throughout their adventure they encounter a healthy dose of fantasy adventure mainstays: giants, sirens, skeletons - it is even revealed that Das Kagel is the mythical Tower of Babel, and host to all manner of horrific entities that predate the modern world. The author seemed to enjoy the occasional tangent in decorating the work with references to implied time travel and immortality that were never really explored further or explained, something I consider a bit of indulgence on his behalf.
Second comes the story of the monks at the monastery at Das Kagel. Their Oracle has died and they await the arrival of the new one, which must travel a long distance. Should it not arrive in a timely fashion, all manner of horrors encased within Das Kagel may burst forth, and the monks have started noticing the emergence of peculiar creatures and hauntings. Worse, if the Oracle does not soon turn up, then the structural integrity of the Gland of Mercy may be irrepressibly damaged, exposing reality to terrible privation. What is the Gland of Mercy? I won't detail it here because it may dull the edge of the first read. Safe to say that any interruption to its workings would be a Bad Thing.
This storyline I also enjoyed. It is host to several engaging characters and responsible for genuinely thoughtful and curious worldbuilding, encased in political intrigue and scheming. There is a slight sojourn during which two of the main characters go on a pilgrimage to a nearby monastery, which is host to a painting by Bosch. I'm not convinced this was at all necessary. It had no measurable impact on the plot and has use mainly as an extended semi-fourth-wall-breaking fan reference to the paintings.
The last of the storylines concerns a woman near Meg, who lives in a village adjacent to the monastery at Das Kagel. She is the lucky recipient of the standard fare of fantasy villager misfortunes. Her son has been imprisoned, her husband is abusive and useless, and her friend has recently been burned at the stake. There is also the unfortunate development of "woebegots" and "filthlings" infesting the village. These are just names for 'things the author found in Bosch paintings that he wanted to turn into characters'.
I did not find this storyline interesting in the slightest. Neither did it seem particularly meaningful to the wider plot. It is the weakest part of the novel.
On the topic of weaknesses: the denouement almost in its entirety. Follett's gang of mercenaries slims at almost comically rapid pace - characters are written off carelessly, inexplicably, or unsatisfactorily. It is as though the author was bored of them and felt rushed, a night-before-the-deadline race to the ending. It's a terrible shame, because it undermines the plot as a whole. The monastery sub plot comes to a mostly satisfactory conclusion, and the Meg one ends pointlessly as it started, but I really hoped for a solid ending for what I thought was by far the most interesting story and character set. It is the one thing that prevented me from rating the novel more highly. Discarding the adventurers so rapidly and without care cheapens the adventure.
I would recommend the novel to fans of dark fantasy. If nothing else, it'll scratch an itch for spooky, horror-inspired fantasy with elements of reality. But it's a shame the plot started off strong and never truly concludes, as though someone truncated the last quarter of the novel proper and what we're left with is someone quickly tying off loose ends.