I don't claim it as an optimal method with regard to yield/time, but sometimes I like to run Aetherochemical Research Facility. It gives 150 tomes, and while you can't tab out to Netflix or anything, it is a pretty easy and fun dungeon if you're practiced at it.

An alternative is to do whatever Crystal Tower raid is on your Wonderous Tails journal that week, for an easy 100 tomes and stickers for Khloe while you're at it. I'd say it's pretty close to the faceroll simplicity of Castrum/Prae, too.

In both cases, I've had decent luck getting newbie bonuses as well.