The books are probably just a more organic type of weapon. Like to blend in with the surroundings. I mean nobody would think anything of a guy carrying a math textbook... until you belt some poor sap over the head with it. And then you probably also have the choice of bringing it to to class and maybe getting a reward for getting to class on time and prepared. Because think about it, possibly that mission where you have to retrieve Gary's (I think) books, maybe you don't really have anything in the way of weapons yet. You and your fists against 3-4 bullies and possibly any prefects that spot the commotion, would be pretty outnumbered, hence the math book as melee weapon. You know use what you got...
