The first thing I'd say is, give it a chance. It does look puzzling at first; the narrative techniques he is using are not immediately clear. But they become so, pretty quickly, and it is worth sticking with the story through the initial puzzlement because it soon becomes very moving.
You might also want to look up "bardo"; leastways, I'd never heard of it. It turns out to be a Buddhist concept meaning "an intermediate, transitional, or liminal state between death and rebirth". I'm not altogether certain that this is exactly what it means here, because there is no hint of rebirth; rather, the bardo seems to be a sort of waiting room for people whose remains of consciousness won't allow them to admit to themselves that they are finally, irrevocably, dead.
Indeed this refusal seems wholly understandable, for at the back of this whole novel is the intrinsic unfairness of death, the arbitrary wiping out of a conscious, feeling personality. None of the souls we meet in the bardo (which seems to be bounded by the cemetery fence) had finished with life: all had reason to want to stay in the world of the living. Hans Vollman was looking forward to consummating his marriage when a beam fell on his head; Roger Bevins, just after he slit his wrists following an unhappy affair, realised the extreme beauty of the world and how much more he craved of it. Elise Traynor, dead at 14, is tormented by thoughts of the sexual and emotional life she never lived to have: "and the choise being made, it would be rite, and would become Love, and Love would become baby, and that is all I ask". Which doesn't seem much to ask.
If death is unfair on the dead, it is equally so on the bereaved, and this is where Lincoln comes in, for his 11-year-old son Willie has just died, and even in the middle of a war, with people criticising his policy and vilifying him personally right and left, this is a catastrophe to put all others in the shade. The action moves between real life, in the days just before and after Willie's death, and the world of the bardo, the souls hanging on at the edge of consciousness. The narrative in the bardo is carried by the speech or thought of several different narrators, while that in real-life consists of extracts from historical books and papers (most are factual, a few are not). One thing this technique does is to show how subjective is historical truth; there is one short but telling chapter about a particular occasion, a party at the White House, which consists entirely of quotes from different guests about the moonlight that evening. There was no moon at all, or a crescent, or a full moon; it was silver, golden, green, blue…. not for nothing do policemen say there is no one less reliable than an eyewitness.
Lincoln, musing in the cemetery where he comes to visit his son's coffin, articulates the pain that has always occupied human minds: " Trap. Horrible trap. At one's birth it is sprung. Some last day must arrive. When you will need to get out of this body. Bad enough. Then we bring a baby here. The terms of the trap are compounded. That baby also must depart. All pleasures should be tainted by that knowledge. But hopeful dear us, we forget."
When the souls in the bardo are troubled by the thought that they might actually be dead (they prefer to think of themselves as sick), they cite their own consciousness in support of their belief: "To whom do you speak? I said. Who is hearing you?" This form of words recurs, and it puts me in mind of a poem of Paulus Silentiarius in the Greek Anthology, a universal epitaph which takes the form of a dialogue between the dead person (or perhaps the words on his tomb) and the living reader of those words. It ends with a question from the living person to the dead:
Who are you that speak,
To whom do you speak?
The poem is bleak, implying that what one was in life matters very little after death. The novel on the other hand seeks for some redemptive factor in the "horrible trap". It might be the sense Lincoln's own grief gives him of communion with the grief of others:
His mind was freshly inclined toward sorrow, toward the fact that the world was full of sorrow, that all were suffering, that whatever way one took in this world, one must try to remember that all were suffering (none content, all wronged, neglected, overlooked, misunderstood) and therefore one must do what one could to lighten the load of those with whom one came into contact, that his current state of sorrow was not uniquely his, not at all, but rather, its like had been felt, would yet be felt, by scores of others, in all times, in every time.
It might also be Roger Bevins's ecstatic sense of the smallest details of life, which causes him to manifest as a being with innumerable eyes, ears, noses and hands, the better to experience them, or the way in which Betsy Baron, alcoholic and inadequate parent, eventually manages to see herself clearly. When her form flickers between all the things she was in life plus those she never attained – "attentive mother, mindful baker of bread and cakes" – there is a sense that these things too were a part of her, even though they were never manifested in life. At the end, one of the bardo-ghosts, temporarily inhabiting Lincoln's body along with him, slips for a moment into that of his horse and immediately feels at one with the animal as well. It is this sense of shared consciousness, experience, destiny, that most lingers from the novel.