If you want to know what makes FreeBSD tick then there's nothing like "Design and Implementation of the FreeBSD OS." Second edition, first edition is good too but only covers up to FreeBSD 5, so it's a bit old now.
And you probably also want to get "TCP/IP Illustrated". All three volumes. Not specific to FreeBSD but contains a wealth of information about TCP/IP, you're going to need to know a lot about how it works. And volume two deals with BSD socket programming.
Hey, wait, just to destroy your brain further, I suggest you also read all the relevant RFCs, starting with rfc793
But seriously,I don't know of any BPF-specific book, but if you're not too skilled then the Unix Network Programming volumes series used to be the go-to books, but I don't know how easy they are to obtain in paper form as they're probably 20+ years old (a guess?)
They give you a good basis for programming the network because FreeBSD (and other BSDs) is the closest thing to Unix[tm] these days.
this was the only one I could find with decent reviews, after watching some of his other stuff he seems like a good resource .. my only thought was .. how much of this would translate to bsd.. as its aimed directly at a linux kernel.
Ouch that's a steep price to pay for something that might be too linux-centric. I don't know what Linux kernels do, but don't they implement bpf with lots of extensions and call it ebpf? How close to bpf is ebpf , I guess, is the $90 question.