The best way is, of course, official documents, but there are actually some books that are worth recommending in addition to official documents.
Operating system principle:
The design and implementation of the FreeBSD operating System.chm
The design and implementation of the 4.4BSD operating System.chm
System Management and configuration:
Absolute.BSD.The.Ultimate.Guide.to.FreeBSD.pdf
Absolute.freebsd.the.complete.guide.to.freebsd.2nd.edition.pdf
The Complete FreeBSD 4th edition.pdf
FreeBSD 6 Unleashed.chm
Network_administration_with_freebsd_7.pdf
BSD UNIX Toolbox---1000+ Commands for FreeBSD, OpenBSD, and NetBSD power users.pdf
Programming:
BSD Hacks.chm
BSD Sockets programming from a multi-language perspective.chm
Security and application:
Mastering FreeBSD and OpenBSD Security.chm
Building a Server with FreeBSD 7.chm
Building.a.high-performance.computing.cluster.using.freebsd.pdf