If you activate Curious Homunculus's mana ability in response to its triggered ability, you can spend that mana to cast an instant spell during your upkeep after it has transformed into Voracious Reader. If you don't spend the mana, it will be lost before your draw step begins; you can't use it to cast the card you draw or to cast a sorcery.

If you don't have enough instant and/or sorcery cards in your graveyard as your upkeep begins, Curious Homunculus's last ability doesn't trigger. If you don't have enough as the ability resolves, the ability has no effect.

If an instant or sorcery spell has {X} in its mana cost, choose the value for X first, and then reduce the cost by {1}. For example, Dark Salvation with X chosen as 2 costs {3}{B} to cast if you control Voracious Reader.

Mana from Curious Homunculus's first ability can be spent to cast a spell that has generic mana in its additional cost, such as Borrowed Malevolence with both modes chosen. It can't be spent to pay costs during a spell's resolution, such as the cost Convolute prompts a player to pay.

Abilities that reduce the cost of a spell apply after abilities that raise them. For example, Borrowed Malevolence with both modes chosen costs {1}{B} to cast if you control Voracious Reader.