S'nice to be able to see the numbers on the enemy's life bars, though. About the only thing useful it does, unfortunately.
The costs on books are flipping ridiculous. Having numbers on the life bar is seriously not worth it

That being said, there's not terribly much worth while to
do with cash, from what I've seen. Shop bought weapons aren't really worth it. I guess the compo item shops might be useful occasionally... but, iirc, by the time you get to the compo shop you're already picking up second+ tier compo items, making the shop mostly useless (Except for a stop-gap thing until you can grind up the better ones, I guess.). So there's no real reason not to spend the cash, really. Might as well spend it on some numbers.
DOUBLEPOSTPREVENTIONEDIT:
Finally got vampire card + sonic punch. Boxer is, at last, a fully functioning member of the party instead of the dangling stick-puppet I use to draw fire. Takes so freaking long to get that bloody card

For those wondering what the vampire card entails, my boxer is now
basically soloing the entire castle gate stage. Before, he could barely survive wading through one or two screens*.
*
When I actually let him fight instead of dangling him upside down over the hordes of grey walkers and spinny things, rendering 90% of the stage impotent vs sniper fire.