[Chaos]

Unless you're a certain kind of Detective, a Book is not the kind of weapon you want to swing at people with. It's weight is less of actual weight (though some cases might say otherwise) and more of how dumb it is to use a book like you would a club.

You could also see a Book's attacks as utilizing spells from the book itself, but that should take much more effort than swinging a sword. Hence, an unwieldy weight.

The Battle Weight part, I figure, could be:
1. Casting takes up your attention quite a bit, leaving your focus on your surroundings and enemies at a sub-par level
2. You like/have to stand still during casting, which doesn't do any favors towards trying to avoid a dagger in your gut
3. You have forsaken exercise so much, it takes more effort to avoid something, and you tend to rely on calculated casts rather than skilled tosses
....and so on and so forth.

OOCly, I see no reason why books shouldn't tell a mage to start lifting. It's part of why dumping STR is not a good idea. (Yes, Fire is tied to STR, but not all mages need Fire ATK to begin with) Considering that spellcasters never have to worry about durability to begin with, I don't think the book weights are unfair.