Translation:Please read the letter immediately.
I hope someone more qualified will comment here, but I'm guessing it's one of those times when Japanese uses a different form than we would. I think "read" here is not transitive, so it doesn't form a direct object, and probably translates more like, "the letters, be reading soon please." Anyone, am i close? :-)
Not quite, 読む(よむ) is still a transitive verb, but using は to make the object into a topic emphasizes the word. It's kind of a subtle difference, but think of it like "As for this LETTER, please read it soon" whereas てがみを直ぐに読んでください would be more like "please READ this letter soon"
It's something you'll get the hang of if you listen to more conversational Japanese. In the big picture, the meaning is quite similar.
'This chair' or 'That chair' can not always be used interchangeably. 'This' refers to something you're near, while 'that' refers to something further away. Or when comparing two things you could use those words to differentiate between them. But there is sometimes indeed some ambiguity between them.
I think there is a slight difference between them. Sure they all mean "at this point in time", but depending on the urgency of the situation you could use different ones. I would use "Immediately" when every second counts and you cant afford to lose any. And I'd use "right away" or "straight away" when something has to be done now, but it's no problem if I first take a sip of tea before I start. But in the end the context makes it clear how urgent the situation is, and they can be used interchangeably.