These two tweaks should improve it (by preventing the largest text and book graphic from overflowing the viewport horizontally on some smaller screen sizes):

  diff --git a/index.html b/index.html
  index a0f978c..040e50e 100644
  --- a/index.html
  +++ b/index.html
  @@ -21,3 +21,3 @@
         h1 {
  -        font-size: 3.8em;
  +        font-size: clamp(2.2em, 7vw, 3.8em);
           margin-bottom: 0;
  @@ -60,2 +60,3 @@
         .hero-img {
  +        max-width: 100%;
           transform: rotate(2deg);
  

Also, I really like what I see in your book so far and I am already learning things on the first few pages. Thanks!