Apologies if this has been asked before but.....

Is it necessary to do the atma/book grind to get weapons for certain classes? I don't want to invest the time to do it.

It's not really necessary to do Atma/Animus grind but if you doesn't do raids and want to get better weapons, you might want to get one for yourself. Alternatively you can spend your time on leviathan EX instead for ilvl 95/100 weapon.

Of course it's not necessary :) There will always be ways to get 'good enough' weapons in the game, depending on what content you do. But you don't really have to invest a whole lot of time into it, I dont get the people who spent weeks just grinding out fates and the others who complain about low drop rate, the whole point of the system was to take a while since people complained there wasnt enough to do in previous patches. You can just go do some fates occasionally when you have nothing else to do/waiting on queues, buy books when you have the myth to spare.