The app contains Dead Tomorrow, Peter James' Sunday Times bestseller from his Roy Grace series of detective novels, also released in paperback and audio forms last week. It's rapidly climbing paperback bestseller charts as I write. The app is £4.99, €5.99, or your local iTunes equivalent, and with it you can also purchase and read the other four books in the Roy Grace series. If you're unfamiliar, it'll let you read the first Roy Grace book, Dead Simple, absolutely free of charge so that you can get up to speed.
Peter James is one of the UK's best known thriller writers, and his Roy Grace series is hugely popular. The books are the perfect thing to have on your iPhone or iPod Touch for the commute, nighttime reading, or even just for a quick diversion when queueing at the supermarket (just like Eucalyptus, if you quit in the middle or reading, the app reopens right at the page you left off).
The folks at Missing Ink Studios have done a lot of great work on this app, and it looks fantastic. As well as the books (well worth the price of admission by themselves), there are lots of great extras to explore, including including a Q&A with James, his research notes, excerpts of the original manuscript with editorial notes, and links to Roy Grace's favourite records playlist.
Really though, you should just check it out in the App Store and buy it now - you won't regret it.