You can copy a spell from your own Spellbook into another book—for example, if you want to make a backup copy of your Spellbook.
This is just like copying a new spell into your Spellbook, but faster and easier, since you understand your own notation and already know how to cast the spell. You need spend only 1 hour and 10 gp for each level of the copied spell.
If you lose your Spellbook, you can use the same procedure to transcribe the Spells that you have prepared into a new Spellbook. Filling out the remainder of your Spellbook requires you to find new Spells to do so, as normal. For this reason, many wizards keep backup spellbooks in a safe place.