The Design and Implementation of the FreeBSD Operating System, 2nd ed.
As a long-time user of FBSD (since 4.x), the book looks like it's quite comprehensive.  Keeping up with all the recent changes is difficult, and this detailed reference covers an impressive range of topics (as far as I can determine from the preview), particularly newer and updated features like the Capsicum framework, security features, ZFS, geom among others.<p>One disappointment is the book doesn't have much to say about UEFI and Secure Boot, which will be important to easily integrate FBSD with other systems on current PC hardware.  Understandable though, as they're still in development and the authors not yet ready to discuss it.<p>For an 800+ page book the price is rather modest.  Looks like one that will have to go on my "must get" list.