Indeed and there is a new edition of the Design and Implementation updated for FreeBSD 10/11 http://www.freebsdnews.net/2014/04/02/design-implementation-freebsd

Indeed and there is a new edition of the Design and Implementation updated for FreeBSD 10/11 http://www.freebsdnews.net/2014/04/02/design-implementation-freebsd-operating-system/ http://www.freebsdnews.net/2014/04/02/design-implementation-... NetBSD is pretty approachable too. With the rump kernel you can run drivers in userspace, so you can use a normal debugger and so on, and not crash the OS you are running.