Ellen Moers in 1970 describes Dreiser as
a writer of massive stature, fierce, unpredictable, eccentric, with the disagreeable habits that have always afflicted major novelists... He was a great novelist, perhaps the greatest of the Americans, and left a reputation for not knowing how to use words... Unusually cerebral in his approach to fiction, he spent most of his time informing himself about everything that science and philosophy could teach the novelist about the act and the will; but he left a reputation for being stupid, unlettered, a 'primitive'.
Of course he's not stupid or any of that -- and Moers is on his side, having "discovered" his fiction "with astonishment" not long before she began writing her very solid book, Two Dreisers, from which this description is taken.
It's the self-taught, wide-ranging aspect of Dreiser I most admire: there are some great advantages to a writer in not being thoroughly steeped in institutional education. Dreiser had limited schooling and in his early days it was in a German-language Catholic school (he had the strange circumstances of being born into mixed Catholic and Mennonite parentage); later he did one year of college paid for by one of his teachers. The rest, he did himself, through library access and building up his own collection of books. You can read about his personal library -- and check out a list of titles -- here.