I think a better system than rewarding extra tomes would be a commendation type system. Receive 1 'new commendation type' for every dungeon you run with a new player. Add 100, 300, 500, 1000, 1500, 3000 rewards.

I feel it'd be more enticing to a lot of players in the long run - adding extra poetics or eso onto a run? Au Revoir, Les Felicias.