Faith in Mind, With a Guide to Ch'an Practice, by Master Sheng-Yen

This is a long poem and longer commentary, but it might be worth printing out and reading if you can't bear to read it online. It starts off:

The Supreme Way is not difficult
If only you do not pick and choose.
Neither love nor hate,
And you will clearly understand.
Be off by a hair,
And you are as far from it as heaven from earth.
If you want the Way to appear,
Be neither for nor against.
For and against opposing each other
This is the mind's disease.
Without recognizing the mysterious principle
It is useless to practice quietude.