By which I mean that Vidal is a great novelist, and Lincoln is an exceptionally good book
The book, luckily, is not concerned exclusively with such details; Vidal has a sure instinct for exactly how much period color to include. He can't avoid some of the other common flaws of historical novels: characters who have only a couple of character traits and keep repeating them from scene to scene; and horrible expository dialogue meant to educate an ignorant audience about their country's history (I include myself) - here is an example from when Lincoln arrives at the White House early in the novel:
"Last time I was here it was 1848." Lincoln looked about with some curiosity.
"Your friend Mr. Polk was in residence then."
Lincoln nodded. "But never friendly to me, particularly after I attacked his Mexican War."
"Ah, the irrespressible speeches of one's youth!" Seward made a comical face.
"You'll be hearding a lot about that speech of yours before you're done."
Lincoln grimaced. "I know. I know. Words are hostages to forture. The only problem is we never know in advance just what the fortune is."
Luckily, such stilted sequences are fairly rare. (I can't wait for people to start writing historical novels about our times: "Ah, yes, I remember the crucial evidence revolved around a blue dress owned by the young lady in question, stained with President's own semen. Strange: rumor has it that the old boy always had a hard time finished using that method, but the woman was apparently rather gifted in this respect." "Isn't it ironic - as the old song goes - that his ejaculate, when finally released, proved to be his undoing?" "You can say that again, homeboy.") The flat characters appear to be a bigger problem. Almost everyone in the novel is a conglomeration of a few traits: for example, whenever Chase appears, he exhibits some combination of these factors: bad eyesight, a love for autographs, a strict desire to maintain his rectitude in all government matters, and presidential ambitions. That's pretty much all there is to him - and plenty of major characters have even less going on.
At first this struck me as a major flaw, but I eventually realized that Vidal was actually doing something very canny. With the exception of a few characters who are too stupid to merit his attention - McClennan, for example, and Sprague - Vidal enters into the minds of virtually all the major characters, and makes them seem like fairly basic, comprehensible people: their thoughts run without fail on only a few rails; Seward, for example, has imperial designs, and that's virtually all Vidal allows him to think about. There is only one exception: Lincoln. We never get to enter into his mind at all; we only see him as observed by these various flat characters, none of whom can really understand what he's up to - and by dividing up the perspective in this way, Lincoln starts to achieve a mysterious grandeur. It seems paradoxical that a novelist can make a character more psychologically interesting by dwelling only on what is observable, but that is exactly what happens here. This opacity also makes certain scenes - such as Lincoln on his son's deathbed - much more moving: first because we know that they actually happened, and second because, by refusing to provide any interior details, Vidal forces us to make the imaginative identification ourselves. He accomplishes something similar with Kate Chase, the second most interesting character in the novel and a sort of a female foil to Lincoln, who is the only other major nonstupid character whose thoughts are never revealed. (I think a great historical novel, incidentally, is waiting to be written about her marriage to Sprague and later life.)
The book is not flawless. It is far too long, with whole chapters that could have been cut with little loss: everything involving David Herrold, for example. The book's vitality depends entirely on its central character; as soon as it moves away from him, it loses momentum, and it is telling that once Lincoln dies Vidal barely has the energy to slap an ending on the novel. Also, on the level of individual sentences, it isn't very well-written. There are all sorts of knotty constructions and loopy double negatives (I would bet anything that the book was written by hand) which a careful writer would have caught: "Several imprudent answers occurred, as always, to Chase and, as always, were replaced with that habitual prudence for which he was never entirely not admired." I have never not entirely understood that last part myself - or have I?
Finally, though, prose is secondary. Vidal possesses a much more important talent for a novelist; he has a natural sense of structure and pacing, and he knows how to manage an immense canvas. He also has a wonderful wit, which is not the same thing as being funny. Here's a sample quote from Seward - "I believe that every young man should live for as long as possible in Paris, in order to perfect his French and strengthen his morals, which is more easily done in a capital where vice is not only everywhere but so repellent that no temptation is possible." This is not going to to make anyone laugh out loud, but it produces a little internal grin, and it's the sort of thing that sustains a reader through a long book. Too many comedic writers today - perhaps taking their cues from television - try to tell actual jokes, and there is something frantic and degrading about this, especially when the jokes fall flat, which some invariably will.
Finally, Lincoln is also a decent guide to the period's history, and a wonderful compendium of political wisdom. Here's one of my favorites, about responding to smear attacks:
Currently, the press was making much of the fact that while viewing the dead on a battlefield, Lincoln had asked Lamon to sing him some ribald songs. The story was curiously repellent; and so believed by many. But Lincoln would not ready any version of the story, much less answer it. "In politics," he had said to Seward, when the subject came up, "every man must skin his own skunk. These fellows are welcome to the hide of this one. Either I have established the sort of character that gives the lie to this sort of thing, or I haven't. If I haven't, that is the end."