In the 1992 edition of Dinosaur Systematics edited by Carpenter and Currie there are quite a few mentions of Monoclonius and discussions over similarities between it and other species as well as analyses of the skeletons placed in that genus. The articles in the book, especially that of Dodson, which mention Monoclonius take a scientific approach to the subject and are very well written in general. The best articles, though, are the older articles which assume that Monoclonius is its own genus, in terms of understanding the animal itself. In fact, one of the best papers for understanding Monoclonius is Barnum Brown's 1914 article.
A Complete Skull Of Monoclonius, From The Belly River Cretaceous Of Alberta was Brown's title and the paper was highly detailed in relation to the find. Brown explains the finds of other scientists, compares Monoclonius to other Ceratopsians, and at the end of his paper announces that three species are to be recognized based on their straight horn (M. crassus), backward curved horn (M. dawsoni), and forward curved horn (M. flexus) with their supraorbital horns also figuring in to the equation of what kind of animal they are.
CM Sternberg, in 1938, compared Monoclonius and Centrosaurus skeletons a second time after Brown. Unfortunately the article is in JSTOR, and, as I've lamented many times before, I do not have a way to get into JSTOR articles. I assume this would be a very interesting reading beyond the first page though, so if anyone can get into JSTOR and pass along the knowledge, that would be great!