In Pathfinder (and is D&D 5 the same?), do Wizards and other casters using spellbooks need to have their medium of preparation out to cast a spell, or are their prepared spells selected from the spellbook so they don't need it at-hand that day?
Clerics pray at the start of the day, Wizards memorize. In Pathfinder, as well as most D&D versions and expies--the Wizard needs the spell book for the memorization, not for the actual casting. Clerics need their holy symbol as a matter of faith after the praying, Wizards won't need their spellbook until they have to memorize again the next day.
Everyone is going to answer the same, because it's in the text over several different versions including specifically Pathfinder, but, I thought I would also provide an example of how it can inform gameplay. In one campaign I ran, the person that hired them took the spellbook as collateral--so the party had to get it done in one day if they wanted the Wizard's spells, which they absolutely needed.
Wizards need their spellbook to prepare spells - they don't need it to cast them. From the wiki:
... he is required to study it every day in order to prepare his spells.