You can keep the tome without killing anybody by pickpocketing the scroll off the priestess, but I don't know of any way to do it without any form of dishonesty at all.
What LG doesn't know won't hurt them. Tons of ways to role play your thief taking it on themself to steal that book and gift it to your non-LG divine spellcaster.
If your Charname is a paladin you don't really need it so no big deal not getting it...
I remember one mod that gave tou the option of cautioning the Umberlee preistess not to use a book from a good faith in her ritual. Forget which one though!
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If your Charname is a paladin you don't really need it so no big deal not getting it...