For a host of reasons, one of the books that I constantly find myself returning to is this wonderful volume by the Zen master, Suzuki Shunryu. There are a lot of profound insights provided by these collected lectures at the San Francisco Zen Center. But my favorite is:
If your mind is empty, it is always ready for anything; it is open to everything. In the beginner's mind there are many possibilities; in the expert's mind there are few...this is also the real secret of the arts: always be a beginner.
Great advice, which in practice is difficult, but in attainment is profound. These lectures are worth reading over and over, as each time we find newer depths to the insights offered by Master Suzuki.