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FreeBSD 
A Comprehensive Guide to FreeBSD FreeBSD Developers' Handbook
FreeBSD Handbook The Linux+FreeBSD mini-HOWTO
FreeBSD System Programming (N. Boeger, M. Tominaga) FreeBSD Man Pages
FreeBSD Porter's Handbook The FreeBSD Corporate Networker's Guide
The Design and Implementation of the 4.4BSD Operating System (M. McKusick, et al)