* {
    box-sizing: border-box;
    font-size: 24pt;
}

.container {
    position: relative;
    margin: auto;
    width: 50%;
    height: 100%;
    background-color: lightgray;
}

.book1 {
    position: absolute;
    top:50px;
    width: 100%;        /* 100% of parent */
    background-color: mediumslateblue;
}

.book2 {
    position: absolute;
    top: 100px;
    width: 100%;
    background-color: rgb(237, 235, 100);
}

.book3 {
    position: absolute;
    top: 150px;
    width: 100%;
    background-color: deepskyblue;
}