[quote=@CaptainManbeard]
I think the exact function differs depending on which source material is used, but generally speaking, Wizards have to read their spellbooks and store the written incantations in their mind for later use, and have to repeat this every day to keep the spells fresh in their minds. They have no natural inborn magical capability on their own and have to study for years in an academic setting to learn their craft. 
[/quote]

More specifically, (5e) Wizards only need their spellbook to learn new spells, change their prepared/memorized spells, or recover a portion of their arcane energy while short resting. If they do lose their book, they still have all the spells they memorized beforehand locked in and can copy them into any new spellbook they create/acquire. Losing a spellbook is devastating, but not fatal. Even so, any half-decent Wizard creates a few backups or dozen down the line and shoves them in a good hidey-hole in case they need them.