While we are recommending and un-recommending resources https://romanzolotarev.com/bsd/ has been my go-to.

joshua stein's interest in getting OpenBSD running on anything he can provides useful insight too https://jcs.org/tagged/openbsd

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If you want a book, Absolute OpenBSD is good though a bit out of date now.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.

Dang, I didn't know that. I thought it was decent enough to get people exposed to doas(1), pkg_add(1), syspatch(8), etc... from which they could then read the man pages.

Thanks for pointing this out and I appreciate the other recommendations in this thread. Massive +1 for @jcs. That dude is awesome and puts out great stuff.