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HTML 

              
                <div class="widget-wrap">
  <h1>JS BOOKS LIST</h1>
  
  <!-- (A) BOOK LIST -->
  <div id="wrap">
    <div id="list"></div>
    <div id="dummy" onclick="bl.toggle(true)">ADD BOOK</div>
  </div>

  <!-- (B) BOOK FORM -->
  <div id="form"><form onsubmit="return bl.save()">
    <div id="fClose" onclick="bl.toggle(false)">X</div>
    <input type="hidden" id="fID">
    <label>Title</label>
    <input type="text" id="fTitle" required>
    <label>Author</label>
    <input type="text" id="fAuthor" required>
    <label>ISBN (optional)</label>
    <input type="text" id="fISBN">
    <input type="submit" value="Save">
  </form></div>
  
  <!-- (X) VISIT CODE-BOXX -->
  <div id="code-boxx">
    Visit
    <a href="https://code-boxx.com/simple-books-list-javascript/" target="_blank">
      Code Boxx
    </a> for more details.
  </div>
</div>
!

CSS 

              
                /* (A) SHARED */
#dummy, .rDel, .rEdit, #fClose, #form input[type=submit] { cursor: pointer; }

/* (B) BOOKS LIST */
/* (B1) LIST WRAPPER */
#wrap {
  max-width: 500px;
  margin: 0 auto;
}

/* (B2) SHARED ROWS */
#dummy, .row {
  padding: 15px;
  border-radius: 10px;
}

/* (B2) ADD NEW BOOK */
#dummy {
  text-align: center;
  font-weight: 700;
  color: #7e7e7e;
  border: 2px dashed #aaa;
}

/* (B3) LIST ITEMS */
.row {
  display: flex;
  align-items: center;
  margin: 15px 0;
  background: #fff;
}
.rDel, .rTxt, .rEdit { padding: 5px; }
.rDel, .rEdit { font-size: 20px; }
.rDel { color: #9f1d1d; }
.rTxt { flex-grow: 1; }
.rTitle { font-weight: 700; }
.rAuthor {
  margin-top: 5px;
  color: #7e7e7e;
  font-size: 14px;
}

/* (C) BOOK FORM */
/* (C1) FORM WRAPPER */
#form {
  /* (C1-1) FULL PAGE */
  position: fixed;
  top: 0; left: 0; z-index: 999;
  width: 100vw; height: 100vh;
  background: rgba(0, 0, 0, 0.5);

  /* (C1-2) CENTERED */
  display: flex;
  justify-content: center;
  align-items: center;

  /* (C1-3) HIDDEN */
  opacity: 0;
  visibility: hidden;
  transition: opacity 0.3s;
}

/* (C1-4) SHOW */
#form.show {
  opacity: 1;
  visibility: visible;
}

/* (C2) FORM */
#form form {
  width: 400px;
  padding: 20px;
  border-radius: 10px;
  background: #fff;
  position: relative;
}

/* (C3) FORM FIELDS */
#fClose {
  position: absolute;
  top: 0; right: 0;
  padding: 10px;
  font-weight: 700;
  color: #fff;
  background: #9f1d1d;
}
#form input, #form label {
  display: block;
  width: 100%;
}
#form input { padding: 10px; }
#form label { padding: 10px 0; }
#form input[type=submit] {
  margin-top: 20px;
  color: #fff;
  background: #2142bb;
  border: 0;
}

/* (X) DOES NOT MATTER */
/* PAGE & BODY */
* {
  font-family: arial, sans-serif;
  box-sizing: border-box;
}

body {
  display: flex;
  align-items: center;
  justify-content: center;
  background: url(https://images.unsplash.com/photo-1608476268141-cb7054ceb2da?crop=entropy&cs=tinysrgb&fm=jpg&ixid=MnwzMjM4NDZ8MHwxfHJhbmRvbXx8fHx8fHx8fDE2NTgzMTUwMTc&ixlib=rb-1.2.1&q=80);
  background-repeat: no-repeat;
  background-position: center;
  background-size: cover;
}

/* WIDGET */
.widget-wrap {
  width: 800px; 
  padding: 20px;
  border-radius: 20px;
  background: rgba(255, 255, 255, 0.7);
}

/* FOOTER */
#code-boxx {
  font-weight: 600;
  margin-top: 30px;
}
#code-boxx a {
  display: inline-block;
  border: 0;
  padding: 5px;
  text-decoration: none;
  background: #b90a0a;
  color: #fff;
}
!

JS 

              
                var bl = {
  // (A) INIT
  data : null, // books list
  hList : null, // html books list
  hForm : null, // html book form
  fID : null, fTitle : null, fAuthor : null, fISBN : null, // html form fields
  init : () => {
    // (A1) GET HTML ELEMENTS
    bl.hList = document.getElementById("list");
    bl.hForm = document.getElementById("form");
    bl.fID = document.getElementById("fID");
    bl.fTitle = document.getElementById("fTitle");
    bl.fAuthor = document.getElementById("fAuthor");
    bl.fISBN = document.getElementById("fISBN");

    // (A2) LOAD ENTRIES
    bl.data = localStorage.getItem("books");
    if (bl.data==null) { bl.data = []; }
    else { bl.data = JSON.parse(bl.data); }

    // (A3) DRAW ENTRIES
    bl.draw();
  },

  // (B) TOGGLE FORM
  toggle : id => {
    if (id===false) {
      bl.fID.value = "";
      bl.fTitle.value = "";
      bl.fAuthor.value = "";
      bl.fISBN.value = "";
      bl.hForm.classList.remove("show");
    } else {
      if (Number.isInteger(id)) {
        bl.fID.value = id;
        bl.fTitle.value = bl.data[id].t;
        bl.fAuthor.value = bl.data[id].a;
        bl.fISBN.value = bl.data[id].i;
      }
      bl.hForm.classList.add("show");
    }
  },

  // (C) DRAW BOOKS HTML
  draw : () => {
    let row;
    bl.hList.innerHTML = "";
    bl.data.forEach((book, i) => {
      row = document.createElement("div");
      row.className = "row";
      row.innerHTML = `<div class="rDel" onclick="bl.del(${i})">X</div>
      <div class="rTxt">
        <div class="rTitle">${book.t}${(book.i?" ("+book.i+")":"")}</div>
        <div class="rAuthor">${book.a}</div>
      </div>
      <div class="rEdit" onclick="bl.toggle(${i})">&#9998;</div>`;
      bl.hList.appendChild(row);
    });
  },

  // (D) SAVE BOOK
  save : () => {
    // (D1) GET DATA
    let data = {
      t : bl.fTitle.value,
      a : bl.fAuthor.value,
      i : bl.fISBN.value
    };

    // (D2) UPDATE DATA ARRAY
    if (bl.fID.value=="") { bl.data.push(data); }
    else { bl.data[parseInt(bl.fID.value)] = data; }
    localStorage.setItem("books", JSON.stringify(bl.data));

    // (D3) UPDATE HTML INTERFACE
    bl.toggle(false);
    bl.draw();
    return false;
  },

  // (E) DELETE BOOK
  del : id => { if (confirm("Delete book?")) {
    bl.data.splice(id, 1);
    localStorage.setItem("books", JSON.stringify(bl.data));
    bl.draw();
  }}
};
window.onload = bl.init;
!
999px

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