I would say tomes are more rare than a visor, but I hope you can do a trade where both side would be pleased.
If I'm not on TR'ed toon or in a raid, I'm always up for some nice EE quests, I'm sure, these days easy to fill any EE GH quests, so only thing to do if you want a dream visor, to invite some friends or do an LFM. It's true, if you have no luck, maybe you will get a tome instead of a visor!!
(I got my +5 tome in EE ADQ1 side chests farming on a +2 loot weekend, using jewel of fortune. Got upgrade tome from +3 to +4 via chest farming as well. But I still prefer to group up, and do quests, instead of farming solo. If you have any experience, where to find +4 tomes most often, don't be shy, share it!!)