George Macdonald was a fantasy writer cited as a major influence by C. S. Lewis. Phantastes's narrator turns twenty one. Part of his inheritance is his father's desk, with a hidden compartment containing a fairy who tells him he shall find the way into fairy land.
The way starts in his room, and he meets is a girl who tells him "Trust the Oak. Trust the Oak, and the Elm, and the great Beech. Take care of the Birch, for though she is honest, she is too young not to be changeable. But shun the Ash and the Alder; for the Ash is an ogre, -- you will know him by his thick fingers; and the Alder will smother you with her web of hair, if you let her near you at night."
This is wise advice.