Anything covering American Christian history from the Second Great Awakening to the Third, or the restoration movements of the 19th century in general, will have some useful information. It just depends on how serious the essay has to be.
If all you're expected to do is to write a brief polemic against Millerism, you might find most of what you need in Anthony Hoekema's The Four Major Cults: Christian Science, Jehovah's Witnesses, Mormonism, Seventh-Day Adventism. I say you might because it's been many years since I read it, and I don't remember how much he focuses on Miller and Millerism as opposed to contemporary Seventh-Day Adventism.
You'll also want to throw in some pious remarks about how no man knoweth the day and the hour.
Assuming you want to write a more serious essay, you'll have to dig a little deeper.
Since the Millerites don't really exist under that name, you'll want to look into the history of the Seventh-Day Adventists and the Jehovah's Witnesses. Books written by people outside those groups will be more useful. You might want to look into the general religious milieu from which Millerism emerged, and in that respect you might find books about early Mormonism useful if they're written by non-Mormons, since Mormonism emerged from the same milieu, so the background information is the same. The reason you want books by outsiders is that if people see a movement as divinely inspired, they see it sort of in "divine time," and it's hard for them to be historically objective. Not always, but usually.
Some books that would be useful is you can find them in your school library or someplace -- I don't know if any of them are in print -- might include:
Anglo-American Millenialism, from Milton to the Millerites, by Richard Connors and Andrew Colin Gow.
Till Morning Breaks: A Story of the Millerite Movement and the Great Disappointment, by Elaine Egbert.
Millenial Fever and the End of the World: A Study of Millerite Adventism, by George R. Knight.
Come Out of Babylon: A Study of Millerite Separatism and Denominationalism, 1840-1865, by Arthur David Tallmadge.
The Burned-over District: The Social and Intellectual History of Enthusiastic Religion, by Whitney Cross.
For balance, see if you can locate Miller's Memoirs, Wm. Miller's Apology and Defense, and some sympathetic writings by Seventh-Day Adventists.
