This article assumes you're migrating from a WordPress.com blog. You can transfer all of your email subscribers with the help of the folks at WordPress.com.
First, login to your new Book Host website and make sure you activate the Jetpack plugin (on the "Plugins" page, click "Activate" under "Jetpack"). Then, follow the instructions to connect your WordPress.com account. You need to enter the credentials for your old WordPress.com blog.
Then, go to Jetpack's website and fill out their Contact Support form. Give them the URL of your old WordPress.com blog and the URL of your new blog at Book Host. Tell them you want to move your subscribers over and they will do it for you.