Fritz E. Froehlich & Allen Kent 
Froehlich/Kent Encyclopedia of Telecommunications [PDF ebook] 
Volume 7 – Electrical Filters: Fundamentals and System Applications to Federal Communications Commission of the United States

"The only continuing source that helps users analyze, plan, design, evaluate, and manage integrated telecommunications networks, systems, and services, The Froehlich/Kent Encyclopedia of Telecommunications presents both basic and technologically advanced knowledge in the field. An ideal reference source for both newcomers as well as seasoned specialists, the Encyclopedia covers seven key areas–Terminals and Interfaces; Transmission; Switching, Routing, and Flow Control; Networks and Network Control; Communications Software and Protocols; Network and system Management; and Components and Processes."