Alexander of Lycopolis: Of the Manichaeans
This shorter work is another refutation of the Manicheans from the Eastern part of the church, and as such is worth reading. It's also kind of fun to see a pretty early version of the idea that "book learning" will lead you away from the faith and into heresy, "those who are devoted to dialectics... sagacious in handling nice and subtle questions; so that now they come forwards as parents and originators of sects and heresies." (pg 241)
Seriously, this is good one and worth attention and reflection.