Depends on whether you're looking for philosophy or energy work. I like the philosophical books listed here so far.
For energy work it's hard to beat Mantak Chia's "Awaken Healing Energy Through the Tao" or any of his other books. http://www.amazon.com/Mantak-Chia/e/B000APK3LG
It's like the difference between Sutra (study) and Tantra (action), both are equally important depending on where you are and who you want to be.