As I have often stated, in modifying cars I think by far the best approach is to measure what you have and then make specific modifications, testing each one as you proceed.
But what about a vehicle (including a trailer) that is being designed from scratch?
I think in that case the best approach is to look at the lowest drag vehicles that have ever been run on roads - and that's solar race cars.
The Leading Edge (Tamai) is I think the gold standard book - but it's also pretty complicated.
But there is one book that has an excellent, short and accessible chapter on ultra low drag aero. It's
Speed of Light - the 1996 World Solar Challenge. It was published by the University of New South Wales (here in Australia) and the cheapest price I can find it for is in the link (Australian dollars).
The chapter on aero design is only 16 pages long but it covers the real essentials, including:
- attached flow
- wetted area
- boundary layers
- surface finish
- induced lift and ground interference drag
- wing tip drag
- frontal area
- interference drag
- ventilation drag
- side wind stability
- pitch stability
The chapter is by Clive Humphris, who was a Ford Australia aerodynamicist, designer of two solar race cars (including a second placegetter) and also co-designer of a world champion ultra mileage vehicle.
He summarises the best approach as series of rules, eg the second rule is that wetted area should be minimised while ensuring flow remains attached.
So if you're interested in developing a low drag vehicle from scratch (and none of the vehicles mentioned in the chapter have a Cd higher than 0.125!), this might be a book to look at.