If you have zero or 1 Spell Cards in your Graveyard, you cannot activate “Hidden Book of Spell”.
When you activate “Hidden Book of Spell” and your opponent chains “Disappear” or “Graverobber”, and 1 of the targeted Spell Cards is removed from play, the remaining Spell Card goes back to the Deck and the Deck is shuffled.
Netrep Q&As:
Q: Is “Hidden Book of Spell” a card that must select 2 cards (in other words, can I activate this trap when only 1 Spell card exists in my graveyard?) Does this card actually mean “up to 2” cards?
A: Exactly 2 are declared at the time of activation. No more and no less can be chosen. If your opponent activates “Disappear” and chooses 1 of the two spells you had selected, the remaining spell would still be placed into the Deck per the card’s effect. Curtis Schultz, 12/10/2003