Don't worry, everyone has their missed-something-obvious moments in gaming ...

And yeah, that's usually what happens to the Umberlee priestess anyway for most people, because the book is just That Worth It and pickpocketing is just that difficult. If you've only got one Thief they pretty much have to major in trap detection with a minor in lockpicking, so to speak, especially if you ever hope to tackle Durlag's, and that doesn't really leave you much if any room for picking pockets.

IIRC I snagged the book once without hostilities, but that was in vanilla in a party that included both a non-dualled Imoen and Alora (who comes already heavily invested into Pick Pockets and can afford to continue to do so since Imoen's handling the traps and locks). In my current playthrough I have only Safana (who is essentially a carbon copy of Imoen thieving-wise if you get her at level 2 as I did) who hasn't added anything into Pick Pockets, and even with the potion of master thievery and several reloads (a PITA, too, given how much the High Priestess walks around), I just could not manage to snag it undetected and had to go the bloody route.

I wouldn't worry too much about it - you don't lose any reputation, the priestesses (especially the High Priestess) are worth a nice amount of XP, and if you're familiar with Forgotten Realms lore, Umberlee is a Chaotic Evil goddess anyhow (hence the lack of rep loss ... also making it quite puzzling that donating to her temple RAISES your reputation!). If you need to donate for rep later and don't want to have to hoof it all the way to the Friendly Arm, there's always the Helm temple in the other part of BG city.