Heal upgrade from books USUALLY add to the value of the heals, not adding the percentage of the heals, the usual precautions is MULTIPLICATIVE NOT ADDITIVE

For a quick example, if your Mausoleum Mage has the max HP of 50,000 his basic healing would be 10% of that max HP or equal to 5,000 to all ally.

So from the books and the masteries you're referring, those percentage are multiplied to the heal, which is the 5,000

So the calculation of the heal you get is 5,000 * 1.45 = 7,250

Is it too small for the epic book? Well it kinda depends... His heal shines in areas where your team get debuffs, if you read his A3, the heal add an extra 10% from max HP if it's removing at least 1 debuff, and adding 15% turn meter if it removes at least 2 debuffs.

Let's ignore the turn meter addition for now, so if this A3 removes at least 1 debuff the calculation changed. The basic heal is now 20%, from 50k max HP that means the basic heal is 10,000! With the bonus from the books and the masteries it becomes 14.500.

And remember, this is AOE... Plus the AoE debuff cleanser (and if conditions are met, an AoE turn meter booster) in 1 move!

But if you're looking for a pure biggest healer, I would suggest Apothecary, 2 turn cooldown (if booked), 35% max HP of target, and can be critical.