"I will read." Is context sensitive and can mean one of two things. It is either a decision you are making in the immediate present, or, by extension, an act of volunteering to do something.
"I'm bored. I will read to pass the time"
VS
"I forgot my glasses at home and cant read the sign."
"I will read it for you."
"I am going to read" is different in that it is said in reference to a plan already made in the past, even if it is the very recent past. It implies a certain level of confidence in the decision made as well.
Hope this helps :)