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.

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

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.