Is there any way of getting the content of a book?

Yes, use WritableBook::getPages().

PHP:

/** @var WritableBook $book */
$pages = $book->getPages();
foreach($pages as $page){
    $text = $page->getString(WritableBook::TAG_PAGE_TEXT);
    //var_dump($text);
}


To be more specific, if you want the text on page 1...

PHP:

$page = 1;
$text = $book->getPageText($page);
if($text === null){
    //page 1 does not exist or is empty.
}else{
    //do something
}