"I read letters."
No. を is direct object. Do your best guess for particles right now and focus on the vocabulary until it gets more grammar intensive. Once we get more complex structure it will be easier to understand what everything is for.
を is the object marker, while は is a subject marker.
In the sentence "Sam eats oranges", Sam is the subject while oranges is the object.
In this sentence, you are the subject (left out because it's implied) and letters are the object.