Why is the "as" needed ?
(edited) Before possessive adjectives, it's optional in Portuguese:
"as tuas cartas" and "tuas cartas" are both right.
Is "I like to read thy letters" wrong?
well, if you got a time machine and are able to travel to 18 century, yes it will be correct
Can it be translated to both meanings. As in, i enjoy to read your letters, and in , i have not read them but if you let me, i would like to?
For the latter, it would be "Eu gostaria de..."