There are many good parenting books out there. I like the Love and Logic book on parenting. I would really recommend reading Greene's The Explosive Child also. I think it would be of particular use considering your child's aggression.
Real quickly, I know what it is to have an overly aggressive child. I've been all through the hitting, spitting, and putting holes in the wall, all by age 5. I found a very good child psychologist, and his office was a godsend. I don't know how else we would have made it. My son was dxed with adhd, and extreme anger. He did not recommend meds first, but wanted to give therapy a try. The therapy over the years has really helped, and my son is now in middle school, never medicated, and pretty much in control of himself. You need someone though who is an expert in children that young, not someone who just goes through the motions, gives you a dx, and that's it.
My son also has Sensory Processing Disorder. You may want to google and read about it. Children with SPD often have meltdowns due to their senses becoming overloaded. Their sensory system can be very sensitive and things that you may not even notice, such as the buzzing of overhead lights, can irritate them to the point of anger.
Or, perhaps your son is just imitating the bad behavior he saw in daycare. Was he like this before daycare? And, can you get a handle on it. If you feel completely overwhelmed, that's when I would recommend a psychologist.