For BSD (especially OpenBSD) you can pretty much just use the man pages and the online FAQ. They are really good. If you want a book, Absolute OpenBSD is good

For BSD (especially OpenBSD) you can pretty much just use the man pages and the online FAQ. They are really good. If you want a book, Absolute OpenBSD is good though a bit out of date now. A lot of it would still be applicable though, if backed up by the current man pages. Reading undeadly.org is a another good way to keep up on developments.