They're actually quite different:
A Lookbook is a customer page with a personalized URL for each client.
Create the complete look, one customer at a time with a Lookbook. Associates create personalized shopping experiences with custom product collections, on pages uniquely prepared for each customer’s needs.
Suggested Video: How to Create a Customized Lookbook
Compose a Message to send your client a personalized email
Messages are a great way to stay in-touch with 1-on-1 emails, or small group emails of up to 20 clients. You attach up to 5 products per email, attach marketing assets, or photos from your mobile device. Composed messages can be sent to any contact in your address book, so long as they have a valid email address.
Suggested Video: How can I attach an asset when composing a message