I know that generally this doesn't matter, but in the case of the spell erase it can matter: Are the spells recorded in a spellbook magical? I know that spells can't be cast from a spellbook like a scroll, but are the spells themselves somehow magical?
A spell in a spellbook is a magical writing, but it is not a magic writing.
To decipher an arcane magical writing (such as a single spell in another's spellbook or on a scroll), a character must make a Spellcraft check (DC 20 + the spell's level).
From Arcane Magical Writings (emphasis mine)
Both a scroll and a spell in a spellbook are magical writings, but a scroll is magic while a spellbook is not. The term Magical Writing refers to a special inscription in a special language or scripting system that is used by magic users. The term Magic is used to denote an object, creature or phenomenon that contains magical energy. Thus a spell in a spellbook is a magical writing but it is not magic. A +1 sword is a magic item but has nothing to do with magical writings. While a scroll is both a magic item and a magical writing.
Thus Erase affects spellbook pages as a nonmagic writing. Compare it to the Read Magic spell that states explicitly that it affects magical inscriptions.