I recently finished reading Paul and First-Century Letter Writing: Secretaries, Composition, and Collection by E. Randolph Richards. The book challenges a number of common assumptions about Paul as a letter writer by drawing on what is known about letter writing in the first century as well as evidence internal to Paul's letters. As the title indicates, Richards spends a considerable amount of time addressing the role of secretaries in the composition of the epistles of Paul.
I found the book to be interesting and well written. Richards' arguments and conclusions have aided and enhanced my understanding and appreciation of a large portion of the New Testament. If you're at all interested in New Testament and Pauline studies I recommend getting a copy of Paul and First-Century Letter Writing.