You don't set the emoji font "as the primary font for the book".
You set the emoji font where the emojis are used.
Import the emoji font file into the ebook.
In your CSS you would then add this...
PHP Code:
@font-face {
src: url(NotoEmoji-Regular.ttf);
font-family: "Noto Emoji";
font-weight: normal;
font-style: normal;
font-stretch: normal;
}
.emoji {
font-family: "Noto Emoji", sans-serif;
}
Then in the body, you would have this...
PHP Code:
<span class="emoji">��</span>