I don't think I've ever gotten a tome higher than +5 in a quest (non-raid). And now they are so rare that they are practically non-existent. It is easier to win the lottery than to win a tome. Yes, some people win the lottery, but most people don't. Drop rates were nerfed very harshly many years ago.
Silver dice usually gives +1 tomes that are useless. Gold rolls tend to provide +5 or +6 tomes, but sometimes, very rarely, you get something better.
If you're looking for tomes playing content, I think your best bet would be in raids. But as othes have said before, even here the drop is low, and you end up sooner farming points and buying a supreme in the store.
I can mention that during the daily dice double xp event I rolled what I believed was a 94 and got a +3 - +4 intelligence tome upgrade, BTA. I was kinda disappointed as I was kind of hoping to get at least a +5 tome or +5 - +6 tome upgrade, but then again I was rolling for XP and everything else was a bonus.