You may have answered your own question. Are their opportunities for another online book rental that the competition has missed, not pursued, or that is specific to your business and not theirs. For instance a particular genre or geographical location etc...

When there is established competition in the area you are considering opening up shop some basic rules come into play.

1. Securing your piece of the market is typically more difficult and costly.

2. You must find and exploit the niche, products, pricing or whatever it is that makes you stand out, why you are different and what do you offer they don't.

3. The competition will not just hand over their market share! Be ready for a fight, your competition is likely to use direct competitive tactics depending on how much of a market threat they perceive you to be.

Good luck!