Food for thought, with heavy spoiler blocks just in case:
* Almost every example in the books I researched for the game had Harry getting off about four or five spells before the description starts to talk about his fatigue, as a general rule. Longer fight scenes, he pushes to his absolute limits, which suggests consequences to me. I suggest you look at the White Night Ramirez/Harry vs. Vampires fight and check out what Harry's done when he gets to the line, "...and all the fighting I'd done, I was beginning to bump up against my physical limits. My legs wobbled, and my spring was more like the lazy, hot, and motionless end of summer."
You'll note it's not as many spells as you probably think.
* In big fights, Harry uses his force rings, which cost him no immediate resources. In later books, these have multiple uses per session. Enchanted items are a smart wizard's friend.
* Harry has been using sponsored magic in the form of hellfire and soulfire for several books now. One of the benefits of sponsorship is that the sponsor can pay the costs for you. That gives him the ability to do extra stuff without taxing himself, at the cost of sponsor debt. After the line I quoted above, note that he brings Hellfire to the party pretty much *every* time he does anything magical for the rest of the fight. This is one reason why it's difficult to be a "pure" wizard and play in higher leagues of supernatural power.
Hope that helps clear up some stuff about how the novels influenced the final design.
-Lenny