Autoresponders are useful in many ways.
Beyond sending email newsletters, or 'ezines', you can use them for follow-up sales messages, customer care - 'O hope you are enjoying your purchase...' etc.
Depending which service you use, you may beable to add an eBook as an attachment, though be wary of size, and bear in mind that most folk don't like receiving attachments.
Better way is to send a message with a link to the eBook hosted on another server - your website.
If people fill in a form on, say, your blog, to receive the eBook, you can have this add them to a list at your auto... account. This would trigger an auto message to be sent with the link in.
similarly as a first point of contact with a link to your blog, podcast etc. If the service allows the sending of .html messages (most do) you can include an RSS button for them to subscribe to your feed.

An excellent service (there are many) is Vertical Response
It is free to start building your list and to start sending messages, you simply purchase packages of a number of messages. With no monthly charges, it is much cheaper for a smaller list whilst you grow it. you can also send your list postcards you design on-line, and they produce & send them for you. Excellent service.

Private Reply to Andrew Barnes (new win)