I bought & read The Design and Implementation of the 4.4 BSD Operating System, it is heavy going, & quite pointless, I found out, unless you intend on modifying kernal source code.
Get one of Micheal M. Lucas 'Absolute' books - I have both first & second edition OpenBSD's, & I will buy the third if it comes about, for a few ££'s, well worth the money to someone who wants to
use OpenBSD.
P.S.
Unix Tools is also worth getting if you want to use the command line.