While I begin to build a collection of information on these topics, if you happen to have clicked here I highly recommend you meet with a trusted trainer or behavior expert in your area. 

CHOOSE A LOCAL, KNOWN COMMODITY.  CONTACT YOUR VETERINARIAN AND OR CALL A FEW VETERINARIANS IN YOUR AREA FOR AGGRESSION RECOMMENDATIONS.  DOGS DO NOT GET MANY CHANCES FOR YOU TO TO THE RIGHT THING! 

Choosing a good trainer/behavior expert that is close and convenient often gives better results than online shopping for behavior help.

If you have questions about aggression, please don't wait. 

Some behavior resources*

GOOD PAWS DOG FORUM

HSUS

IACP

IAACP

some basic thoughts on dog to human aggression

Many dogs use a series of assertive seeming behaviors to maintain distance from unknown (or untrusted) visitors.  Barking, growling and snapping are common ways to maintain distance, and are rewarding because they often maintain distance.

As a rule of thumb, I assume the barking is about mistrust and work from that perspective almost automatically.  I tend to incrementally work dogs closer to the initial triggers, causing success+duration at easier increments before challenging anything close to the real triggers.

In the 80s and 90s, I'd have interpretted this same barking as some form of unacceptable protective behavior and formulated a correction strategy to deter the barking, growling, lunging, snapping, etc.   Once I had the behavior under control, I would then try to introduce the fear inducing object of attention carefully and in a favorable manner.

Now, I see that if I do not "correct" the barking and snapping and lunging, etc, I have less overall stress to overcome.  My previous approach often worked, but in the end, it worked as a result of stress reduction relative to the aggression trigger.

Introducing the aggression triggers at far enough distances or in small enough increments allows a gradual increase without having to first cause added stress via corrections.

When a gentle, trust provoking approach will not work, the behavioral roots are strong, sometimes genetic, sometimes lack of important socialization and bite inhibition training during puppyhood.  Occasionally, a physical malady can be the cause.

I am not much a believer in the "there were no signs" story of dog aggression.  Historically, when faced with such ideas, good trainers identify many unnoticed signals.  If your dog behaves assertively to maintain distance with untrusted people, what happens if someone closes that distance quickly?  Unexpectedly?  Children trip and fall on dogs.

If you've read this much, please know I think you should seek help, internet experts are a dime a dozen!  Free advice is generally worth about that much.  :)

*NOTE: I list a variety of viewpoints because I have a variety of viewpoints and I think that as you look for behavior help, you should look for models of training you are apt to follow up with...