index.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width">
    <title>repl.it</title>
    
    <link href="./css/style.css" rel="stylesheet" type="text/css" />
  </head>
  <body>
    <header>
      <img src="./icons/logo.png" alt="logo">
      <button id="newBook">+</button>
    </header> 

    <div class="library">
  
        
    </div>

    <form action="" id="form" class="closed">
      <h2>Add a new book:</h2>
      <label for="author">
        <input placeholder="Author" id="author" type="text">
      </label>

      <label for="title">
        <input placeholder="Title" id="title" type="text">
      </label>

      <label for="number">
        <input placeholder="Number of pages" id="number" type="number" >
      </label>

      <label for="read"> Read
        <input id="read" type="checkbox">
      </label>
      <br>

      <input id="btn" value="ADD BOOK" type="button">
      <input type="button" value="CLOSE" id="close">
      
      <p id="warning">
        
      </p>


    </form>

    <script src="./js/script.js">
    </script>
  </body>
</html>