Yes there is an order to some of his books. The John Corey series starts with Plum Island, the Lion's Game, Night Fall and his last one in this series is Wild Fire (I haven't seen this in the UK yet).
The Paul Brenner series starts with The General's Daughter and next is Up Country.
Other than that, they are all fine to read in whatever order you like. Charm School is great but my favourite DeMille book has to Plum Island, though I love all the John Corey series. In fact I love his books full stop but oe or two of the early ones are not as good, they just don't keep you.