One of my favorite classes in uni was rhetoric, and one of my favorite books to this day is Farnsworth's Classical English Rhetoric. I highly suggest it as a tote around, open to a random page and read as much as you like and never be disappointed reading.
http://www.amazon.com/Farnsworths-Classical-English-Rhetoric...
Regardless, the one that I spent a good year reading through multiple times was Farnsworth's Classical English Rhetoric. A brilliant book that challenged my knowledge of rhetoric and enabled me to hone my rhetorical skills. You can read it to learn or read it just for the awesomeness of the passages.
http://www.amazon.com/Farnsworths-Classical-English-Rhetoric...
Recommend: Ward Farnsworth.
Apparently some law school dean who has written some books on law, none of which I have read. I personally just picture him more as a classicist.
His grasp on English writing knocks me off my feet. It is seriously something to behold.
His book on explaining chess tactics (also available online free[0]) is an absolutely amazing display of effective communication in writing.
In another writing style, he has three books[1][2][3] and apparently an upcoming fourth[4] where he presents an idea/concept, cites meaningful examples in the wild of their use, and provides his own commentary to touch up on his chosen topics. They make for delightful reading.
[0]: chesstactics.org [1]: Farnsworth's Classical English Rhetoric [2]: Farnsworth's Classical English Metaphor [3]: The Practicing Stoic: A Philosophical User's Manual [4]: Farnsworth's Classical English Style