This is the kind of thing that throws me, and I guess some other students, off. I understand that -아요/-어요 ending can be versatile, but to show it at this point of the course without mentioning beforehand and while people are still getting used to the different endings we just learnt (imperative, propositive, interrogative)... doesn't really do any good.
Yea, I believe so. I read that pronouns, including he/she/you are not used very often. Instead of he/she/you, the person's name is used instead. If the topic was previously established as person's name/he/she/you etc, for example if someone said, "Tom reads a book" before, then this sentence could be translated that way. "He reads the most interesting book."
But since we don't have any context, my default is to translate it as "I".
You've already gotten good info, but to track on to this, any language that tends to omit subject usually relies on context. If it is the first utterance in a conversation, subjects are much more often used, as are when changing subject, but it is then dropped after the conversation continues unless it changes again.
Actually, literally translated this says "reads the most interesting book". If u translate this into English, you have to add "I" for it to make sense. therefore you'll have to conjugate the verb to agree with its subject, making it now say "read" instead of"reads". So really, this sentence is not a command. In korea, people would never use this sentence to order someone to read a book. So, since the subject is omitted in this sentence, we assume that there was some form of context in a previous sentence stating the subject. Since we dont know what the subject is, the default is to just assume it to be "I"
This doesn't use the proper grammar form. "Let's read" and similar sentences that suggest a course of action taken by the speaker and the listener use the ㅂ시다 ending, as in 읽읍시다 (let's read). In this case, 가장 재미있는 책을 읽어요 is just "(implied subject) reads the most entertaining book."