Why is the accusative necessary? "li" is the direct object, not "libro".
"Ŝi legis lin kvazaŭ libro." = "She read him as if she was a book."
"Ŝi legis lin kvazaŭ libron." = "She read him as if he was a book."
Ŝi legis lin kvazaŭ (ŝi legus) libron
Man, I hate that the past tense for read is spelled the same way as the present tense ugh