It will teach you the semantics of the posix interface and introduce you to some canonical utilities. That information is relevant today (we still use sed, make, etc..). Given the choice, I'd probably opt for a book specific to linux, freebsd, etc.. kernels alongside something like unix power tools.
UNIX Power Tools might give you some ideas.
Learn the Linux shell. People will think you're some kind of coding ninja :)
http://www.amazon.com/Power-Tools-Third-Shelley-Powers/dp/0596003307