How do I write a letter to an author?

I really want to write an email to my favorite author on everything having to do with nuclear and atomic energy. However, I have no clue how to write it to sound at least semi peofessional while also showing my excitement for his books.

All I have so far is:

"Dear Dr. Mahaffey (his name is Dr. James Mahaffey),

My name is [my name]."

And that's it :|.