I think a natural choice is large provides like Amazon.
However, if you want to sell eBooks on your own and maintain all of the revenue, then WooCommerce and Easy Digital Downloads would make excellent options.
Both software packages are WordPress plugins and they make it very easy to deploy an e-commerce store for digital goods.
Both plugins have strong development teams behind them and they have a slew of independent freelancers who can offer assistance if needed.
I've used WooCommerce myself for multiple years and we've deployed many WordPress websites that use it. It has hundreds of extensions you can add on to it for maximizing the potential.