I list below the best books you can use for design and understand the wetland systems (buy online onhttp://www.amazon.com/ orhttp://www.ibs.it/):
Constructed Wetlands in the Sustainable Landscape
(Craig S. Campbell, Michael Ogden)
Treatment Wetlands, Second Edition
(Robert H. Kadlec, Scott d Wallace)
Biogeochemistry of Wetlands: Science and Applications
(K. Ramesh Reddy, Ronald D. DeLaune)
Wetland Plants: Biology and Ecology
(Julie K. Cronk , M. Siobhan Fennessy)
Wastewater Treatment in Constructed Wetlands with Horizontal Sub-Surface Flow (Environmental Pollution)
(Jan Vymazal, Lenka Kröpfelová)
Small Scale Constructed Wetland Treatment Systems: Feasibility, Design Criteria and O&M Requirements (Werf Reports)
Efficient Management of Wastewater, Its Treatment and Reuse in Water Scarce Countries”,
(Al Baz Ismail, Otterpohl Ralf, Wendland Claudia)