I can't see anything about that in the PHB but it kinda makes sense...
I don't really understand why that is (except to prevent everyone from carrying around a book of scrolls as my player intends)
The magical writing explain how to draw in the energy (whatever it is) and direct it in the form of the spell (a projectile in the case of Magic Missile and Fireball), but if you have deciphered the Wizards writing then you can perform the somatic, verbal and, if necessary, material components...why can't you use the Spellbook to cast spells (providing you have appropriate spell slots left)
From PHB pg 178
Once a character deciphers a particular magical writing, she does
not need to decipher it again. Deciphering a magical writing allows
the reader to identify the spell and gives some idea of its effects (as
explained in the spell description). If the magical writing was a
scroll and the reader can cast arcane spells, she can attempt to use
the scroll (see the information on scrolls in the Dungeon Master’s
Guide).
Wizard Spells and Borrowed Spellbooks
A wizard can use a borrowed spellbook to prepare a spell she already
knows and has recorded in her own spellbook, but preparation
success is not assured. First, the wizard must decipher the writing in
the book (see Arcane Magical Writings, above). Once a spell from
another spellcaster’s book is deciphered, the reader must make a
Spellcraft check (DC 15 + spell’s level) to prepare the spell. If the
check succeeds, the wizard can prepare the spell. She must repeat
the check to prepare the spell again, no matter how many times she
has prepared it before. If the check fails, she cannot try to prepare
the spell from the same source again until the next day. (However,
as explained above, she does not need to repeat a check to decipher
the writing.)
From SRD (also DMG pg 213)
Spell Completion: This is the activation method for scrolls. A scroll is a spell that is mostly finished. The preparation is done for the caster, so no preparation time is needed beforehand as with normal spellcasting. All that’s left to do is perform the finishing parts of the spellcasting (the final gestures, words, and so on). To use a spell completion item safely, a character must be of high enough level in the right class to cast the spell already. If he can’t already cast the spell, there’s a chance he’ll make a mistake. Activating a spell completion item is a standard action and provokes attacks of opportunity exactly as casting a spell does
Note that in order to cast a spell it either must be via using a spell slot (prepared or spontaneous) and the other prerequisites must be met (rested, etc.)
A scroll is a spell completion item (that it is mostly completed and only the finishing touches remain to cast it - all xp and material components are already paid in the making of the scroll).
A spell book is magical writing but is not a spell completion item.