There really isn't a "BSD OS" anymore, today's BSD derivatives are based upon the indeed 4.4BSD & 386BSD releases, but they have each diverged considerably.
They do still share code, how much is unclear, but plenty of design changes have been made that render that book obsolete.. with OpenBSD for example, the developers consider the kernel source itself the definitive source of up-to-date documentation.
Reading the books can't hurt, so I'm not going to recommend that you don't.
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