In my opinion, a decent cartridge library will have about 50 books spread out across the various field of ammunition. It is quite possible to have several hundred books on ammunition. If your interest is really in a study of ammunition, it is impossible to have too many reference works.
It is, though, always best to start with the basics.
The down side is that books on cartridges, and guns, are coming out at a lightning pace these days, and the prices on them are usually very expensive. It is very discouraging. I am just short of fanatical about books on the arms and ammunition subject, have about 1500 in my own library, and I can no longer begin to keep up even with the ammo books. You simply do the best you can.
If your interest is absolutely specialized - whatever field of ammo you have chosen - and you don’t care about the rest (and that is NOT a criticism), try as you can afford to do it, to obtain everything in print on your field alone, and don’t worry about the rest. Believe me, for many fields, that will keep you busy of itself.