Hmm how come the potential stats of Monster book arnt upgrading when your book lvls

I think 15% > 2% atk. Not sure though.

Assuming the 15% is completely applicable (ie. you don't already have above 85% ignore def) you'll get a better than 2% damage increase on mobs with 15% ignore pdr. Lots of regular mobs have 5%/10% and on those the 2% atk is very slightly better (on its own - just pick something with 15%ignore pdr + something else useful and you're good to go)