I know this isn't a D&D world but ogres seemed to be like Ogres are and gnomes are a familiar race though yours are different. And then the undead creatures which were like vampires but you didn't call them that. What went into the descions of using names peopel know verse creating a name like you did the undead?
Well, my experiences are obviously going to be shaped somewhat by D&D, just given how much a part of my life it's been for so long. But when it comes to things like that, I prefer to draw on the various myths/tales that D&D drew on, rather than D&D itself.
So the use of ogres as big, nasty, violent creatures was pretty much a no-brainer, because when you say "ogre," everyone knows at least vaguely what you're talking about. I changed them up in the details a little--making them cyclopean, making their violence driven by their religion, that sort of thing--but I still wanted to draw on the classic ogre trope.
With the gnomes, I wanted something different. I didn't want this world to include the traditional type of goblin/orc villain. Once I'd come up with these creepy, twisted (literally and figuratively), awkward, vile, almost alien little things, using the term "gnome" just felt really appropriate.
As far as not using the term "vampire," I don't really have answer to that except that it felt right. That's obviously what they are, despite differing from traditional vampires in a lot of way, but it just felt appropriate never to name them as such. (I make a lot of my creative decisions by instinct, rather than by process. Not all, not even most, but many.)
Was that iron to wood spell used on the blood to kill the undead evil dude something you thought of or something you saw or heard about? I can see a D&D player argueing with a DM to let that happen.
Fantasy (D&D and otherwise) has often included spells designed to render an enemy's weapon harmless. I'm sure I must have seen metal turned to wood somewhere over the years; I can't think of where, but I don't pretend the concept is original to me. I do like to think that my particular use of it in this fashion was fairly creative, though.
Are you concerned with D&D similiarites? The different circle of spells really seems like spell levels for instance.
Not especially so. I've actually written columns about D&D's influence on fantasy (on Suvudu, not here), and about how many concepts either introduced or embraced by D&D have become fantasy tropes. So on a general level, I've no problem being part of that pattern.
Are there a
few places where, in retrospect, I wish I'd gone in slightly different directions? Probably; I don't want to be pigeonholed as "the D&D guy."

But they're minor details here and there, nothing more--and while any further Corvis Rebaine books will obviously still have them (as will
The Goblin Corps, but in that case, it was quite deliberate), I'll be moving away from them in other upcoming projects, just to keep myself from getting too stuck in any given rut.