Lucifer's Hammer by Niven and Pournelle. The plot isn't worth bothering about, but it's got some very vivid images of the aftermath of a large impact.
More to the point, I believe much of the imagery is based on serious research.
I'm not absolutely sure, for instance, that it is the consensus today that comets, at least when they are as near to the Sun as Earth is, are a cluster of smaller objects swarming around the nominal "point" the object is located at, on the average. They might be more solid than we thought in the 1970s, before any space probes had ever got a good close look at one.
But I'd guess 50/50 they are indeed that diffuse, and that's mixed news as far as Earth goes in case of impact. Because on one hand, Earth won't get hit with all the mass; on the other, what fraction does hit will be spread out in dozens to thousands of separate impacts, depending on how large a fraction of the number of bodies making up the comet head do hit Earth. If comet heads are as diffuse as they thought back then, then even if the center of mass is square on the center, some of it will still stream past; how much does depends on how diffuse the cluster is. But the more spread out it is, the more damage the mass that does hit does because there won't be one big crater, there will be lots and each one does more harm than the same mass would as part of just one.
As someone upthread a couple years ago noted, even if the comet were all one solid body it wouldn't crack open the crust or seriously affect either Earth's orbit or rotation. It would be very bad news for anything nearby and as I said here a long time ago, I don't trust the calculator to properly estimate the degree of damage to the surface ecosystem due to atmospheric shock and still less due to vaporized water and the disruption of weather we'd expect--the major damage done in
Lucifer's Hammer was due to water, first the masses flash-vaporized in the impacts, which generally punch right through to the sea floor, and then even greater masses of sea water vaporized as the ocean pours onto white-hot liquid rock. Of course the water cools the melted ocean floor patches pretty fast but it does so by being vaporized and streaming as high-pressure steam up into the stratosphere, where it flows away as a steam hurricane (maybe a reversed hurricane if hurricanes generally are low-pressure in their centers, so the winds spiral in the opposite direction). A portion of these storms make their way over land, and drop the massive oversaturation of water there, making for a spectacular Deluge. There's also a lot of dust from both land and sea impacts, and so when the massive rains finally peter out there's a Fimbulwinter due to the atmosphere being more opaque for some years.
I've already guessed what I think would happen to the world in 1700; it almost doesn't matter if the impact is a near miss with only a relative few of the components of the comet hitting, or it hits square on. The challenge the OP gave, to try and figure where on the globe it would hit, is almost meaningless; parts will hit almost anywhere, as the Earth turns and as objects are perturbed around the Earth by its gravity. It might be possible to estimate a central zone that gets more impacts and a shadow zone that is mostly sheltered, but the damage the comet does is pretty much global no matter where the direct impacts are, since it is climatic. A very light near-miss might leave centers of civilization intact though damaged, a heavy dead-on hit might come close to wiping out life on Earth though it probably won't, entirely--and if anything survives I daresay some humans would be among them. It seems reasonable to me to split the difference and say that most of civilization is wiped out, due to the failure of crops everywhere--first they take a terrible hit in the form of death toll from the immediate Deluge, then the survivors of that wind up mostly starving to death and the cities are abandoned; those who make it through the first year will be living in a disrupted ecosystem where new farms are hard to establish, but will probably be in such low numbers that they can survive by gathering and hunting, and a big portion of surviving humanity may simply switch back to living in such a fashion for centuries to come. By sheer chance, some agricultural centers will be re-established, but these will be too small and few to sustain a serious level of civilization. They might, however, retain books and even literacy, and so be on something of a fast track for rebuilding civilization again. As a wild guess I said that maybe by now, several centuries on, things would be as they were in 1699, but actually I think that must be wrong unless the impacts were quite light--they'd be different. There might be individual city-states or kingdoms rather comparable to stuff that existed after 1000 CE, but they'd be islands in a wilderness of low-population density gatherer-hunter peoples who will not be rushing to join civilization, nor allowing themselves to be displaced easily. They may or may not be interested in trade. They won't be interested in joining anyone's army.
So the civilized places will tend to be rather ugly, being organized around gradually poaching wider lands for their expanding settled population by means of cultural genocide, getting embroiled in frontier wars with gatherer-hunter bands who live there, taking them as slaves and breaking them to a servile role on the farms and in the towns, and taking their land. Maybe some of this process will take the form of bands getting induced into the farm/urban system by means of trade goods and their gradual recruitment as fur trappers and the like and as scouts against other bands; such peoples might retain some ethnic identity and political rights. Or might not. Eventually one urban center's sphere will brush up against another and we probably will see both trade and war between them.
Maybe some will go into ocean exploration and trade, especially if knowledge of how to build 1700-type ships has been retained in some library and a boat-building culture revives soon after the Deluge. But note that in addition to flooding from rain, the impacts will also devastate shore areas especially due to tsunamis, and it will be an odd chance indeed that preserves any seacoast people well enough to be among the handful of post-impact civilizations. It seems more likely to me that the arts of navigation will have to be reinvented from scratch, for the most part, even if there are books that describe an advanced ship in detail, because those books won't describe the crafts of basic shipbuilding that are common knowledge at the docks of major maritime peoples pre-impact. Again books that are dedicated to recording the arts of navigation will probably describe in loving detail advanced techniques of navigation but won't mention the stuff every sailor learns his first weeks at sea. A society that preserves such books and then develops a maritime branch will probably leap forward dramatically eventually, but first there will be generations going out on boats that aren't much better than the most primitive rafts or canoes, until they get the hang of it and start understanding what those old books were talking about.
) But one thing I trust these guys on is their math. Whereas, while I also trust the astronomers who crafted the impact calculator, I am not so sure the calculator does a fair job of computing big impacts. I'm sure it's great for the little ones, but the big ones all come out the same. At no point does it say "the Earth's biosphere is shrouded in a layer of mist and dust that won't let significant sunlight through for 3 years" nor does it say "the hot craters on the sea floor vaporize x gigatonnes of water which rains down on the hemisphere." In short I think it sort of underestimates huge elements of the impact's aftereffects. Or ignores them completely.