Enable the Book resource to track the view (read) progress of each student (user) as he/she reads the pages of the Book resource.
Upon completion, have the Activity Completion flag set to TRUE.

Currently, just viewing the Book resource set the Completion flag to TRUE.
mod/book/view.php (166)

    // we are cheating a bit here, viewing the last page means user has viewed the whole book

    $completion = new completion_info($course);

    $completion->set_module_viewed($cm);