One of my favorites I get every 5 years or so is the Anatomy Coloring Book. Because I am very visual I find working through that keeps my anatomy fresh.
Even though I am not currently doing personal training I do have a copy of the ACE instructor manual, which I find great to dip into from time to time.
I also think it is useful to keep track of what fitness related books are on the best seller lists. Clients are often interested in quality of life, lifestyle choices such as diet, and mind/body issues, in addition to how to get buff or strong. If your clients come to you for an opinion on some current popular book or idea it is good to have some familiarity with these topics. For example I would recommend reading something by Michael Pollan for issues related to food, health, and sustainability.
There was also a book out recently called ‘Buddah’s Brain’ that gives some great insights and techniques from current research in neuroscience on happiness, mood, anger, meditation, and so on.