This book:
Already widely used for Internet services and firewalls, high-availability servers, and general timesharing systems, the lean quality of FreeBSD also suits the growing area of embedded systems. Unlike Linux, FreeBSD does not require users to publicize any changes they make to the source code. Это и многое другое вы найдете в книге The Design and Implementation of the FreeBSD Operating System (Marshall Kirk McKusick, George V. Neville-Neil)