.btnAgregarLibro {
    background-image: linear-gradient(to right, #283048 0%, #859398  51%, #283048  100%);
    margin: 10px;
    padding: 15px 45px;
    text-align: center;
    text-transform: uppercase;
    background-size: 200% auto;
    color: white;            
    box-shadow: 0 0 20px #eee;
    border-radius: 10px;
    display: block;
  }

  .btnAgregarLibro:hover {
    transition: 0.5s;
    background-position: right center; /* change the direction of the change here */
    color: #fff;
    text-decoration: none;
  }         

  .btnEliminarLibro {
    background-image: linear-gradient(to right, #FF512F 0%, #DD2476  51%, #FF512F  100%);
    margin: 10px;
    padding: 15px 45px;
    text-align: center;
    text-transform: uppercase;
    background-size: 200% auto;
    color: white;            
    box-shadow: 0 0 20px #eee;
    border-radius: 10px;
    display: block;
  }

  .btnEliminarLibro:hover {
    background-position: right center; /* change the direction of the change here */
    color: #fff;
    transition: 0.5s;
    text-decoration: none;
  }