The old standards. Stevens to start. If you don't already know Stevens' thoroughly first, I wouldn't go deeper. The kernel is not separable from the userland-f

The old standards. Stevens to start. If you don't already know Stevens' thoroughly first, I wouldn't go deeper. The kernel is not separable from the userland-facing API. I'm not convinced you actually need to / want to understand actual internals. But assuming you do, next is Tanenbaum (maybe start with the newer "Modern" book). "Operating Systems: Three Easy Pieces" also seems well regarded, although I haven't read it myself. The FreeBSD book is great, if you're specifically interested in BSD. There's also a video lecture you can buy on McKusick's site. I had the pleasure of an on-site presentation and it is absolutely fantastic.