(1) Incorporation if not done yet. You need to be a legal entity to deal with other corporations. They need to be able to sue you to trust you. It also creates a liability shield so that you yourself are not sued, just the "company."
(2a) Increase the number of users as quickly as possible through adverts and site links. So forth. Go and win top spot for your service on aggregator sites like lifehacker, gawker, gizmodo, wired so forth.
(2b) Another way to do this is to announce it as an "open source" project so that anyone can offer to add to the database in any language. Simple checks to ensure quality, a lawyer to ensure that no copyright law is broken, letters to publishers stating the same and allowing them to contribute to both benefit (they can get free advertising to read their books and you grow your database). It also increases brand loyalty and calms those upset that their book is not on here. Everything in the public domain should already be in the database through an automated script which copies other sites database, hopefully with permission or in the dead of night (shhh!! - its not illegal). Don't open source the algorithim, thats the money. But do hire out soon to improve it, partner with pandora also possible).
(3) Offer this service as an application on ipad, iphone, and android. This can be enlarged in a number of ways. You can become an add-on to other apps, like a search tool or automatic playlist generator for public domain apps like Audiobooks, and Aldiko. You can become a player app yourself, like pandora. You can be an add-on search to Amazon, Barnes & Noble, mp3 stores for audiobooks; so forth.
(4) Start selling books!! This is where you make money, either selling through an online store of audio books and ebooks.
(5) Sell Out. Sit back on your money. Live the good life, write a book, and one day remember where the plan came from and give a few books that way