So now I'm going to present my simple but elegant solution to the problem. To wit, a new disadvantage (two new disadvantages).
Disciplines of Faith (Arcane Rituals) (-10 points)Your magic requires constant research into the tides of magic, the position of the stars, and stranger things. You must own a spellbook that contains your notes, observations, and calculations on how and when you can cast spells. This spellbook weighs 1 lb per 10 points in spells that you know, and costs $10 per lb. You must study from this spellbook for 1d hours every night - this is time taken away from sleep, being on watch, travelling, etc. You do not need the spellbook on you to cast a spell if you studied it the night before. On any day that you cast a spell, you consume $5 of reagents and spell ingredients - these are fetishes, fine spices, and powered gems that weigh almost nothing. If you ever find yourself without reagents or your spellbook, you are at -1 on all spell-casting rolls per day that you go without either, and -2 per day that you go without both. The penalty is reduced to 0 as soon as you study in your spellbook and have access to reagents again.
Disciplines of Faith (Tome Magic) (-15 points)As above, but your spellbook is a physical representation of your magical ability. It weighs 1 lb per 5 points in spells that you know, and costs $25 per lb. You cannot cast spells at all unless your spellbook is on your person (in your backpack counts, but not in extra-dimensional space such as a Hide-away pouch). You get a +2 on all rolls to cast spells while your book is physically in your hand.
Tome magicians often use their spellbooks as power items. These spellbooks are often ornate or written in faerie ink for the usual CF modifiers (per DF4: Sages).
CommentaryThis is basically Disciplines of Faith (Chi Rituals), recast for use by wizards. It gives the proper feel of wizard, constantly pouring over his spellbook and pulling newt tails or whatever from his pockets. It also gives the GM another option for treasure: other people's reagents let the wizard stock up on long trips, and since reagents have almost no weight, they make for a very portable treasure.
I'm not sure if Tome magicians should be able to make their spellbooks Fine or Very Fine and apply the bonus while they have the book in their hand. It'd be hideously expensive and inconveniencing for a delving wizard, but I do like the idea of a big bad wizard with a huge, bejeweled, absurdly large book of magic that the delver's thief can try to steal away from him. Hmmm.
Hat Tip: Bruno suggested Tome Magic, with the requirement for a costlier book that could be used as a power item.