It's a novel sustained by style, would be my call. Like Vanishing Point (the only other Markson novel I have read), it is composed of roughly a thousand brief observations on when, where, an how various artists died, interspersed with notes on what they said about their art, about critics, about each other, and some (many unattributed) quotations. It's the tour of a sensibility, so it does have a character of a sort in the Writer himself, but what keeps you turning pages is the alertness, intelligence, and inventions of the style.
A lot depends, too, on some ability to recognize and some knowledge of the many, many names that crop up, and various arcana about them -- for instance, on p. 144, Markson mentions that "As a Marine pilot in Korea, Ted Williams several times flew as Colonel John Glenn's wing man." On p. 145, we have the following entry: "The Boudreau Shift" -- that is, radical defensive shift towards the first-base line that the Cleveland Indians, under manager Lou Boudreau, adopted to thwart the left-handed, pull-hitting Williams. Williams refused to do the obvious thing and punch one into left field because, hell, he's Ted fucking Williams. The two observations, a page apart, sketch a cunning little fable about art, artists, and history. You do need to know who Williams, Boudreau, and Glenn are -- but a little Googling and you're there.
I didn't need to look up the unattributed quotation on 189: "If you can do it, it ain't bragging." Dizzy Dean! -- who also shows up on 79 (in conjunction with Marianne Moore) and 133 (in conjunction with Ezra Pound) and no doubt another spot or two, including the final page, where we find out how he died (heart attack). Like ol' Diz, Markson ain't bragging when he claims he can write a novel that isn't a novel at all -- he did it.