Food is an artifact type. Even though it appears on some creatures, it's never a creature type.

You can't sacrifice a Food to pay multiple costs. For example, you can't sacrifice a Food token to activate its own ability and also to activate Maraleaf Rider's ability.

Whatever you do, don't eat the delicious cards.

The Underworld Cookbook's last ability can target any creature card in your graveyard, not just one discarded with its first ability. For example, if a Granite Gargoyle you control dies after being dealt damage by Lightning Axe, you could use The Underworld Cookbook to make a delicious Gargoyle entrée.