BlinkBox Books offers a broad selection of titles which can be read on the related Android, iPhone, or iPad apps as well as on
compatible ebook readers or on Tesco's Hudl tablet. Tesco hasn't revealed just how many titles they carry, but reports have been circulating that Tesco has been aggressively pursuing deals with publishers.
Update: Guess what? Blinkbox Books doesn't support ereaders. I think that's a good reason to avoid the store.
This UK retailer has been working to build a digital content store for several years now. They've bought several companies, including Blinkbox (a movie service), We7 (music), and MobCast (a white-label ebookstore). That last was bought in September 2012, which should give you an idea just how long Tesco has been working towards today's launch.
Tesco has also launched an Android tablet in September 2013. The Hudl, which was developed in partnership with the French tablet maker Archos, was clearly intended to compliment the BlinkBox content store in much the same way that the Kindle Fire tablet complements Amazon's music, video, and ebook retail operations.
As of early December Tesco reported that they had sold 300,000 Hudle tablets in just under 2 months. A second model was rumored to launch in March, but I think that window may have passed.
BlinkBox Books is being launched into a highly competitive market where several strong retailers have already staked a claim. In addition to Amazon, Kobo, Apple, and Google, there are also local competitors like Sainsburys and Waterstones, which sells Epub ebooks in addition to selling Kindles.