Maybe that's the best compromise - as @kellewic said, we know our characters can continue learning even while offline:
When you have your tome and enough Knowledge Points, you can start the learning process: the points are consumed and the book is inscribed, removed from your inventory and added to one of the Learning Slots of your character – if you have at least one not in use. Once launched, a learning process cannot be interrupted and continues even when your character is offline. The inscribed tome disappears when the learning has been completed.
It could be as simple as this: when you go offline, the learning process continues, as stated above. If you go offline while in a city, the process goes slightly faster (~10% maybe?), which makes sense from a realism standpoint as well, since studying is probably easier in the safety and peace of a city than it is out in the wilderness.
While this is going on, your character idles around the city like an NPC, not doing anything important, just making the place feel more alive. There's no benefit to being offline instead of playing, cities look less empty, and players get an incentive to spend more time in cities. I think that could work.