> I loved the first two I read until I read the one about a 'big brother' machine that monitored and decoded all email communications
I think you're talking about "Digital Fortress" ... that one is absolute tripe and has to be his worst book by a long way; but maybe I think that way because I'm a bit of a techie and can see the massive holes and nonsense in it.
By far his best of the five I've read is "Deception Point", which like Digital Fortress is not a Robert Langdon book. All the Langdon books are pretty average IMO, and sometimes I find it hard to willingly suspend my disbelief (e.g. when people keep going after falling out of a helicopter). I've heard is latest book (Inferno) is poor literary fodder but an excellent page-turner, so I'll probably give it a chance this summer. But if you want a good Dan Brown - it's Deception Point in my experience.