The exact means of preparing a given wizard spell shifts from day to day, from place to place, and from person to person (due to a variety of reasons relating to ley lines, weather, individual physiology, etc.)

A wizard's spellbook is a minor magic item which constantly updates itself to reflect what its owner needs to do to prepare a spell at the current time and place.

This is the reason why it takes special materials to write a spell into one's spellbook, why a spell from someone else's spellbook can't be easily prepared or copied, and why a wizard who prepares the same spell every day still can't prepare it from memory.