I'm slightly annoyed by the cap too. I ran each of the new dungeons once. Then I ran "expert dute roulette" two days... I'm not sure what else I did - I wasn't trying to cap - but I did.

I think it should take more effort than that to reach the cap... so either the cap is too low, or they're too easy to earn. I DO see the value in preventing players from farming tomes too quickly. Honestly I'd be okay with receiving fewer tomes - but I felt like reaching the cap made me want to stop playing. Maybe that is the point? I went and got one of my DoH master books instead