Thats not a terrible book, but more meant for beginners this day and age. EV is estimated value. It is a tool used when deciding to make a move of some sort (since its an 'advanced' book it assumes you know what this is, try reading his beginners books first).
For example. If calling a bet costs you $10 and you will win $50 if your hand holds up and have a 50% chance that this will happen, you will profit $40 half the time, and lose $10 half the time, your estimated value then is $30. The math here is simple for explanation, it usually runs much much deeper. This can be applied to many situations in poker. Good luck.
here is a great site that explains it in depth