To an extent you would just have to explain to them that it's still D&D just set in the modern world and with a lot less walking through forests and underground caves. Of course some classes need a bit of explaining (not as many knights these days so perhaps they'd have to be a knight of the cross or some group that was around to lend back up if ever needed?) and others would need a little different mindset (thieves are often treated as socially acceptable in D&D worlds but wouldn't really be so now... though they certainly still exist.)
Frankly, it would all depend on just how close to the Dresdenverse you wanted your game and how adaptable your group was to such changes. But I have to ask, why not lend them your books and/or get them to buy their own?? I've frankly never been in a gaming group that wasn't looking for new/good fantasy books to read.
