This line does not work

$('#bookcontainer').load( startSlide );

while this line does

$(window).load( startSlide );

#bookcontainer is a div containing four images.

Here's my whole code:

// JavaScript Document
$('#bookcontainer').load( startSlide );

var slide;

$('#slider').hover( 
function() {
        $('.arrow').show();
}, 
function() {
        $('.arrow').hide();
});

function startSlide() {
    $('.book').show();
    slide = setInterval(slideR, 5000);   
}


function slideR() {
    $('.book').first().css('left', '960px').appendTo('#bookcontainer').animate(
    {
        "left": "-=960px"
    }, {
        duration: 1000,
        easing: 'easeOutCubic'
    });

}


function slideL() {
    $('.book').last().animate(
    { "left":"+=960px" }, {
        duration: 1000,
        easing: 'easeOutCubic',
        complete: function() {
        $(this).prependTo('#bookcontainer').css('left', '0px');   
        }
    });

}


function right() {
    clearInterval(slide);
    slideR();
    startSlide();
};


function left() {
    clearInterval(slide);
    slideL();
    startSlide();
};