This is the script that I use on my site. You can try adding that for your ebook link. The first set of scripts is for viewing the site when you are not logged in. The second set is for viewing it when logged in to write.as, so you have to replace ‘YourAlias’ with the alias for your blog.
var a = document.querySelector('a[href="https://catholichalos.org/books"]');
if (a) {
a.setAttribute('href', 'https://catholichalos.org/.epub');
}
var b = document.querySelector('a[href="/YourAlias/books"]');
if (b) {
b.setAttribute('href', 'https://catholichalos.org/.epub');
}