First, they delete you for not having a book bag--and, with this "No marriage for you if you haven't served continuously as at least an assistant hounder since you turned 23", this should make you look at the organization as genocidal as well as stealing all your time. That combination can be used to enforce every possible rule--including dictating that it be a "proper" portfolio style bag and not a denim, canvas, or nylon bag. It can also be used to ban or regulate everything--from what color an umbrella you use (I personally recommend the Blunt umbrella in yellow, because it has blunt points for no poking out others' eyes and it is highly visible, but it will probably also be banned), to carrying a flashlight (not trusting in joke-hova).
A book bag serves one purpose, and one purpose only. It helps you manage handling your books in one place. This might be helpful if you have other things to handle at the same time, such as rags, many books, papers, and other items. But, if all you have is 1 or 2 books and the washtowel rag (which can be stuffed neatly inside one of them), a small folder style book holder, or no book holder at all, may be sufficient. While going in field circus, if you still go at all, I recommend not getting too big a bag. Not only it is more difficult to handle, but you could well be finding yourself pious-sneering. You just need to carry your damnation book, several rags, perhaps a few copies of whatever littera-trash you are featuring that month, and your call book and perhaps a few wastes of paper. Do you need a library-sized bag for that? And yes, if you are going in traffic in rainy or foggy conditions, I do recommend a bright yellow umbrella for better visibility.
