Spotify appears to be setting its sights on Amazon in the audiobook department. The company says that U.S. subscribers will be able to purchase and listen to more than 300,000 audiobook titles. Audiobooks represent just 6% to 7% of the overall book market, says Spotify. However, the category is growing at a rate of 20% year over year. Nir Zicherman, Spotify’s vice president and global head of audiobooks and gated content, calls it “a substantial untapped market."