Netflix, Amazon and Hulu are battling it out over digital video while Apple recently threw a wrench into the streaming music market with iTunes Radio.

Now another market is on the brink of exploding as new and existing digital publishing services are looking to extend the subscription economy concept to one more medium: e-books.

Scribd has been around the block for awhile now, serving a popular domain for storing, sharing and reading digital files ranging from SEC filings to selected chapters from The Hunger Games to double as promo materials for accompanying films.

Part of what has made Scribd so popular is that it has been free to conduct these activities from a consumer standpoint.

Now the service is building on that popularity with what looks like the goal of covering every base in digital publishing -- including a new all-you-can-read e-book subscription service.

In a model made familiar by Netflix and Spotify, among many others in digital movies and music, Scribd has unveiled a new scheme offering unlimited access to e-books for $8.99 per month.

Available now with a free trial offer to boot, Scribd's e-book library is supported across iOS and Android devices as well as via web browsers.

Things appear to be off to a solid start as Scribd already signed HarperCollins as the first major publishing partner, bringing on "the majority of the HarperCollins US and HarperCollins Christian backlist catalog."

Altogether, Scribd's extensive digital library hosts more than 40 million books and documents.

While an exact number for e-books isn't being broken out publicly, Scribd touted that it already offers "thousands of best-sellers and new releases in a wide variety of genres." A few of the more well-known mass market titles being advertised on the landing page include 2001: A Space Odyssey, Water for Elephants, and Lemony Snicket's Unfortunate Events franchise.

Scribd isn't alone in wanting to tackle this teeming market.

New York-based startup Oyster is arguably the other biggest name in this area right now given how much buzz it has generated since launching earlier this year.

With unlimited access to more than 100,000 titles and solid reviews abound (ZDNet's Jason O'Grady especially praised the "gorgeous" UI ), Oyster also appears to have a good footing.

But as Scribd co-founder and CEO Trip Adler stressed to GigaOM, his company "already has reach, and the hardest thing is building audiences at scale."

Scale and a familiar brand name are two things that Scribd arguably has going for it at this point in time, giving it a leg up against Oyster and other challengers in the race to become the verifiable "Netflix for books."

As highlighted on the Scribd blog on Tuesday, Scribd's active user base currently stands at approximately 80 million across 100 countries and 80 languages worldwide. And aside from charging $1.00 more for its subscription fee, Oyster also doesn't support as many devices and platforms (at least for now).

But this race is far from being decided just yet.

There are some alternative methods to unlimited e-book rentals. For example, Kindle owners with Amazon Prime can take advantage of the Kindle Owners' Lending Library, which opens up access to more than 350,000 books that can be digitally borrowed for free without due dates.

Stemming from that, there's the age old institution of the brick-and-mortar library itself. Many libraries across the United States (and worldwide) already have digital lending platforms set up for borrowing digital versions of ancient texts to New York Times bestsellers.

The catch with these is that libraries only lend out a few (or sometimes only one) version at a time, just like they would with physical books.

The key to these subscription options from the likes of Scribd and Oyster is that their business strategies are based upon the subscription economy thesis and also the on-demand habit many consumers have come to expect for digital media in general.

Image via Scribd