This is a work of literary criticism.
I've read a great many pieces of lit crit where the author is kinda confused, and criticizing a work as if disagreeing with the critic were a literary flaw. Chesterton is not confused. He lays it out in the opening that he is chiefly concerned with the authors as Heretics -- those who disagreed with him. (He ridiculed the notion of calling one's self a heretic. Everyone ought to regard himself as a sterling example of orthodoxy by the principles he preached.)
One advantage of it is that since he realizes he's doing it, he manages to separate what they are saying with how they are saying it, and can point out the second is brilliant before going after the first with a hacksaw. He also doesn't say anywhere that a writer "failed" to do something that he wasn't even trying to do.
Some of the chapters are on authors still famous: Kipling, Shaw, Wells. Others are lesser known: George Moore, Mr. McCabe, Lowe Dickinson. Still others are on tendencies, such as the chapter on slum writers, where he points out that slum novels always depict it from the upper class point of view -- the slum dwellers speak in husky tones, not the slummers speak in shrill and affected tones. Fascinating analyses of all sorts of novelists of his day.
The conclusion, interestingly enough, ties it back into literature by pointing out that art for art's sake had not produce great works like the writers he discussed who did try to put forth a view, and therefore he urged writers to do so on artistic grounds if no other.