Bards can work very well for this. Whispers is built for it but dark. Lore works really well too, as can Glamour.
Enchantment and charm spells are your bread and butter: Sleep, Charm person, Enthrall, Suggestion, Hypnotic Pattern, etc... There are lots of great stealthy options: Disguise Self, Invisibility, Silence, Non-detection... There are great utility options to get yourself out of particular problems: See Thoughts, Knock, Zone of Truth, etc... Finally, Bards get options for a lot of good detection and scrying spells to help them locate the loot or person they need to find.
Add the skill features they get, JoaT and expertise, as well as naturally high Charisma and you've got a very decent infiltrator, one that can work the social side very well.