why is it meus?
meus, since an adjective, matches with liber in grammatical case, gender, and number (nominative, masculine, singular).
Because liber is masculine. :)
So it doesnt matter if the speaker is male or female? "My" would be fem., masc or neut. depending on the noun "my" describes?
Yes, meus works like other adjectives in Latin and agrees in gender and number of the noun it modifies.
Anybody clear on the difference between libro, librum and liber? Would really help to know that - thanks.