Oyster, an online e-book subscription vendor, now has a half million titles in its catalog and is on a run in making top deals with big name publishers.

The $17 million-funded venture has a short and simple philosophy: read as many books anytime, anywhere. It debuted last year with 100,000 titles and now may run past Amazon's Kindle library, which holds a half a million titles.

The news illustrates more readers than ever are embracing online bookstores and e-reader devices such as Amazon's Kindle and the smartphones being embraced.

"Roughly half of our reading activity happens on phones," says Eric Stromberg, CEO of Oyster Books.

Oyster's library of 500,000 e-books are available for $9.99 a month, with titles from over 1,600 publishers. According to Oyster half of its subscribers are accessing its service using a smartphone during the day hours. Subscribers on weekends and nights tend to use the iPad.

E-reader devices, such as the Kindle, broke the way for electronic reading on smartphones, like the iPhone, as well as tablets, which provide a larger display and are becoming the new 'book' format.

According to Scribd, e-book readers split their reading time between tablets and phones with the average reader using the devices for just under an hour every day. But that doesn't mean they're reading a lot more or even finishing more books as readers tend to finish just one of four books started through Oyster's service.

But whether that's just a generational trend or a digital reading trend is still unknown. Reports indicate more people are using online libraries such as Kindle's Lending Library and more competition in the market is happening and that could spur greater reading activity.

A big growth driver may be the fact that younger readers are jumping on such services. A Pew Research report indicates e-readers are getting younger and mostly female.