The time when a person thinks about becoming a minister will, of necessity, become the first of many times when one will sense a deep need for spiritual reflection and refreshment.
Here is one pastor's recommendation:

Silence. Prayer. The Bible.
Anything by :
Anything by :   - when you need a good laugh, or a good   cry, or both.
Anything by :
The Heart Aroused by David Whyte.
Anything by :    (especially The Unnecessary Pastor!)
Between Gloom and Glory by Glen Miles Silence Silence Silence Silence Silence.

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Rev. Glen Miles, Senior Pastor, Sandy Springs Christian Church

Buechner refers to the works of Frederick Buechner
Annie Dillard is an author who has written several books including Pilgrim at Tinker's Creek
Anne Lamott is the author, among other things, of an autobiographical story of how an alcoholic drug addict found God. "Traveling Mercies" is a must read