Jack by Marilynne Robinson.
So here is John Ames Boughton aka Jack, who you'll recall we met in Home and Gilead and who was an occasional off-screen presence in Lila, and who was in many ways the most interesting character in those books, having a murky past involving various unforgivable misdemeanours, much dissolution and misbehaviour and equally much saintly patience and forbearance from his father and siblings. Well here he is getting a whole book to himself, or mostly to himself.
We're back a few years before the events of Gilead and Home here, probably at the tail-end of the 1940s. Jack is living in St. Louis, holding down a succession of menial jobs which just about allow him to rent a dingy room in a boarding-house. Despite this he occasionally finds himself on the street if his drinking habits get the better of him. As luck would have it he is looking reasonably respectable when he helps Della Miles pick up some papers that she's dropped during a rainstorm, and senses an instant connection. Only one problem: Della is black, and for all that she's highly-educated and with a respectable job as a schoolteacher, that's still a pretty insurmountable problem.
A couple of further encounters and an ill-fated first date later and Della happens to find herself locked in a cemetery at night, and who should happen to be skulking around on one of the benches but Jack, who takes it upon himself to look after her for the night. Luckily Jack is a man of some reading and erudition and is able to engage her in diverting philosophical discussion to while away the long hours until the caretaker arrives to open up the gates in the morning.
And so a tentative romance ensues, though obviously severely constrained by several different factors - obviously there's the whole forbidden inter-racial thing, and it is literally rather than figuratively forbidden, inter-racial marriage being illegal, at least in Missouri, but there is also Jack's own inherent unreliability. Partly there is his weakness for The Drink, but this is really just a symptom of a more general problem: a tendency to deliberately torpedo his own happiness and then try to escape the consequences by fleeing, either literally or by recourse to the bottle. That in turn may be just a symptom of some deeper issues around self-worth and not wanting people to rely on him for anything for fear of letting them down - better to let them down hard and early to get it out of the way.
Jack - the son of a preacher, don't forget - seeks refuge in a church in one of the black neighbourhoods of St. Louis, looking for a couple of things: firstly, some soup, and secondly some sort of homely reassurance that things will be OK. The minister is reluctant to offer this, though, and points out all the ways in which Jack will be ruining Della's life if things continue.
Of course Della is not some unwilling participant in all of this, and has her own challenges to overcome, particularly her own father - also a preacher, as it happens, and also not especially keen on the whole thing, not out of some reflexive hatred of whitey (although maybe a bit of that too) but more that Della will be letting herself in for a world of hurt and anguish regardless of how respectable and upstanding a white man she happens to choose, and a shifty intermittently-employed drunkard is just the cherry on the cake.
While Della is in Memphis dealing with her family Jack does what he does best and flees to Chicago, where he makes quite a nice respectable life for himself for a short while working at a bookshop, until he happens to mention to his landlady that the "wife" he's left back in St. Louis and who may be joining him later is a coloured lady, at which point he is swiftly shown the door. Having pretty much nowhere else to go, he turns up at Della's father's church in Memphis and is taken back to the family home on the understanding that this is a one-off event during the course of which Della will be invited to make an irrevocable choice between Jack and her family.
Since this is effectively a prequel to Gilead and Home, the reader who's read the books in something resembling publication order will already know that Jack and Della don't get to waltz off together into the sunset, maybe to a state with less draconian laws regarding inter-racial marriage, but instead that Jack turns up at his father's house pretty obviously having left somewhere in a hurry at the start of those two books' timeline (this would be at least seven or eight years later), and that later on Della shows up looking for him. So that knowledge colours the reader's perception of Jack, in particular of how it ends.
If you were of the opinion that, instead, you wanted to read the books in the order of when they were set, then broadly speaking Jack and Lila happen around the same time (maybe a year or two apart, it's hard to tell), and then there's a gap of maybe seven or eight years (judging by the age of, for instance, John Ames' son in Gilead) and then Gilead and Home happen roughly in parallel. I'm not sure why you would do this, though, to be honest.
What I would say about Jack, as beautifully written as it is, is that I expected to enjoy it slightly more than I actually did. That's because Jack is an interesting character in the first two books, Home in particular, who you feel you want to know more about and who has some enjoyably spiky interactions with his father and siblings. There is a bit of a repeated cycle in Jack, though, of Jack and Della having some delightful stolen moment somewhere, Jack going back to his lonely room and reflecting that actually she'd be better off without him, resolving to confront her and end it for her own good, and then turning up on her doorstep only to find her waiting to reassure him and persuade him to stick around. A few cycles of this process occupy more of the book than they perhaps should, and things which we know happen (i.e. from reading the earlier books), like Jack and Della having a child, don't happen within the quite narrow timescale the book's narrative occupies (probably a couple of months at most). That it's probably the least satisfying of the four books in the series shouldn't be taken as too much of a criticism, as the standard is very high, and I think if you're going to read one (probably in the order they were published, so starting with Gilead) you should probably read all of them.
I very deliberately choose not to analyse my own thought processes in terms of how I decide which book to read next; if I had done so here it might have occurred to me that the title of this book is a shorter version of the title of the previous book in the series, Jack Maggs. I couldn't swear to it but I think the only other example from this list is the pair of Invisible and Invisible Cities, though they were a couple of years apart. That's if you require the shorter title to appear at the start of the longer one, of course; if you relax that requirement you get things like Home appearing in The Road Home and Dinner At The Homesick Restaurant.
[STOP PRESS: there's also the pair of Transit and Transition]