Process Design Manual for Nitrogen Control (USEPA Technology Transfer, 1975). The most comprehensive treatment of denitrification. Chemical pathways, design elements, modelling parameters, as well as nitrogen control technologies other than nitrification-denitrification. Although dated, this is still the best discussion found and an excellent reference.
Operation of Wastewater Treatment Plants vol. 1 (USEPA Office of Water Programs, 1990). This is the series that most operator use to prepare for the licensing exams required of them. The series does an excellent job of explaining the mechanics behind wastewater technologies as well as providing trouble shooting guides. The series of books is offered through California State University, Sacramento, Department of Civil Engineering.
Barnes and Bliss, Biological Control of Nitrogen in Wastewater Treatment ( New York; London:E & F Spon, 1983). A good source for the theoretically inclined. One of the few sources that provides mathematical models of denitrification.
Metcalf and Eddy, Wastewater Engineering, 2nd Edition (New York: McGraw Hill, 1979). Excellent for nitrification but only adequate for denitrification. Chapter 11-7 covers Bardenpho, oxidation ditches, and has an excellent sample design problem.