A good start insofar as an overview and general critical reception of Chinua Achebe's work, in particular Things Fall Apart, would be to read "After Empire: Chinua Achebe and the Great African Novel" written by Ruth Franklin and published in the May 26, 2008 issue of The New Yorker magazine. Franklin, a noted book critic, is a frequent contributor to The New Yorker and the former editor at The New Republic magazine.
A more specific look at the character of Ezinma could be accomplished by reading "Ezinma: The 'Ogbanje' Child in Achebe's 'Things Fall Apart,'" written by Aron Aji and Kirstin Lynne Ellsworth and published in the journal College Literature (Oct., 1992–Feb., 1993) by the Johns Hopkins University Press in an issue dedicated to "Teaching Postcolonial and Commonwealth Literature." Aji is a professor of Instruction in Literary Translation at the University of Iowa.
A third article to read for perspective on the role of Nwoye is from the archives at California Linguistic Notes, a peer-reviewed journal published at California State University, Fullerton. It is "Fifty Years on: Problematizing the Heroic Ideal in Chinua Achebe’s Things Fall Apart," written by Christopher Anyokwu at the University of Lagos, Nigeria and published in the Winter 2009 issue. The links to all three articles are below. And finally, reading through the works cited pages of the Aji/Ellsworth and Anyokwu articles would yield additional works that may help inform your essay.