One thing? Yikes... too many options to pick from. I'll just list a few.
1- Give all the other characters more personality.
2- Make the bad guys a little more sneaky and less noticeable. Less of "oh, he looks like a shifty character; he's probably a bad guy!" Or "there's something odd about him; he's gotta be bad"
3- I thought the entire defeat of evil in Book 5 was too easy. If it were me, I would have had Malak destroyed but his forces left in disarray so that the good guys still have some work to do for a while. Then again, that might not have fit into the general theme of weapons, so.... oh, well...
4- Maybe it's just me, but I didn't like how Book 1 fit in with the rest of the series. I understand it was an introduction, but it reminded me too much of "Taliesin" from Lawhead's "Pendragon Cycle," where the story focused on the father more or less preparing the way for his son, who has been destined for great things. But I liked how Lawhead did it but not Graham's way. If it were me, I would have just introduced Benji in the first book and made his father less significant.
5- Make the bad guys' evil plan more clearer. The whole flooding of the earth to wreck havoc didn't really seem like a good, evil plan to me. Maybe, because the first book focused so much on the ruling class, the evil plan instead could have been to infiltrate the ruling class and weaken the continent even further in preparation for a massive military invasion. That would have been far more interesting.
"The Christian shoemaker does his duty not by putting little crosses on the shoes, but by making good shoes, because God is interested in good craftsmanship."- Attributed to Martin Luther
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