Just four months after subscription-based mobile ebooks service Oyster finally hit the iTunes (s aapl) App Store, the company has announced a big $14 million investment round, led by Highland Capital Partners, and including participation from existing investor Founders Fund, according to the New York Times.

The new round brings the company’s overall funds raised to $17 million, building off of a $3 million round in 2012. Eric Stromberg, Oyster’s CEO, said via phone today that the money will go directly to building out both personnel and product. Oyster’s app currently offers more than 100,000 across publishers such as HarperCollins, Houghton Mifflin, and others.

“Basically, we saw that there was a lot of existing demand in the market, as well as the demand we anticipate as our service grows,” Stromberg said.

He didn’t share specific numbers about just how large the Oyster user base has grown since its launch for iPhone…