By the way, it is originally in Complete Divine, NOT Book of Exalted Deeds.
And yes it allows you to prepare a spell not in your book, then you simply write the spell down as it is retained in memory until cast and it states you can write a spell you have memorized. A side note..
Say, the Recaster picks an actually useful spell, and records it down in a spellbook. Then another mage can come along and copy it from the Recaster's book, as now it stands as an Arcane Spell. Thus the god of magic can then copy spell, and thus theoretically it would be in the book. (Since you need True Believer to unlock the tome's ability, or have divine spells..)
Right?