I'm a strong believer that your overall gameplan should influence your book size.
An aggressive book that plans to end the game in the first 10 turns probably doesn't need more than 40 cards, and can afford to spend a lot of spellbook points on high cost spells and out-of-school spells. An aggressive book is likely to care more about the mana cost of its cards than the spellbook cost. If there's a cheeper/better card out of school, an agro book may be able to find room to run it, even in multiples. An agro book is in the business of asking questions, and usually there's only so many questions to ask.
Contrariwise, a defensive book needs to have a plan for 20 or more turns, and because control books are in the position of answering questions instead of asking them, they need solutions to a wide range of problems. This means more in-school spells, and fewer expensive splashes from out of school. The defense game plan, though, often includes investments like mana crystals and spawn points, so individual cards can often have higher relative mana costs.
But remember that these are just guidelines. Even for a very focused agro book, it is important to be able to throw a change-up and play a longer pressure game. If your opponent knows exactly what you'll be doing next turn, he'll almost certainly be able to counter. So far, no mage is so overwhelmingly agro that it can afford to broadcast its moves with impunity (though some Forcemaster builds make me wonder).
Likewise, even the most defensive book will need to go outside of its trained schools for some essential cards. No school is broad enough to defend against all threats by itself (though Earth Wizard may come close).