Italian (like portuguese and spanish) usually does not use the pronouns
It is present tense:"I read a book". It isn't grammatically incorrect in English, but perhaps a bit stilted. We don't usually say "I read a book" just out of nowhere, but it could in response to a question. For example, if someone asked how you pass the time waiting for a plane or something, you might say "Oh, I read a book". Or, if someone said "What do you do to learn a new skill?", you would tell them "I read a book"...
Depends on who the subject is in your sentence. "Read a book" is generally considered a command or suggestion, so you would be addressing someone else (either one person, or more than one). Therefore, you would say either "Legge un libro" (You (singular) read a book), or "Leggete un libro" (You(plural) read a book). The conjugations for Italian vebs are quite straight forward and easy to remember, and I recommend learning them. It helps a lot. Eventually, you will recognise who the "who" is in sentences, just by knowing the conjugations.