Well, Zetetic Astronomy: Earth Not a Globe by Samuel Birley Rowbotham is really the definitive FE text, so that should be the first thing you check out. Most of the other books by FE'ers borrow heavily from his work, so it really is the best place to start. I've also read Does the Earth Rotate? and Zetetic Cosmogeny, and whilst interesting, I don't feel they have a lot to say beyond Rowbotham's original ideas.
An excellent book to read in conjunction with Earth Not a Globe is Christine Garwood's Flat Earth: The History of an Infamous Idea. Though it suffers from bias due to Garwood's globularist sympathies, it is nonetheless a well researched and detailed account of the history of FET, and is especially useful for its account of the Universal Zetetic Society and the debates which took place at the time.
Beyond that, James tells me that David Wardlaw Scott's Terra Firma 1901 is very good (though I haven't read it myself).